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Ultrafast Fluorescence Dynamics of Highly Stable Copper Nanoclusters Synthesized Inside the Aqueous Nano-Pool of Reverse Micelles
Abstract
Herein, we have reported a new strategy for the synthesis of highly stable fluorescent copper nanoclusters (CuNCs) with L-cysteine (Cys) as a protecting ligand within the water nano-pool of reverse micelles (RMs). In the present work, efforts are also given to address the origin of excitation-dependent fluorescence spectral shift of CuNCs. From our experiments, we have elucidated that the broad fluorescence from CuNCs in RM consists of two spectrally overlapped bands corresponding to the metal-core and surface states of CuNCs. The intrinsic emission of CuNCs distributed in shorter wavelength regions (<470 nm) is mainly originated from metal-core. On the other hand, extrinsic fluorescence band (>470 nm) is caused by surface states and consists of a much broader emission due to the presence of numerous surface states. The trapping of excited electrons in the various surface states leads to the emission in the longer wavelength regions and is believed to be responsible for excitation dependent emission of CuNCs in RMs. Excited state dynamics, which controls the optical properties of CuNCs, have also been investigated by time correlated single photon counting (TCSPC) and femtosecond fluorescence up-conversion techniques. Femto-second fluorescence up-conversion and TCPSC decay profiles of CuNCs comprise of multitude of lifetime components spanning from <1 ps to few nanoseconds time scales. We have rationalized the dynamics on the basis of several competing deactivation pathways and a broad distribution of radiative electron-hole recombination dynamics originating from core and surface states.
... Recently metal nanoclusters [1][2][3][4][5] (MNCs) have been gaining attention amongst researchers because of their merits such as emissive properties, 6 prominent photostability, a large Stokes shift of Z100 nm, 7 and lack of toxicity 8 for applications such as sensors, 1,2,6,9 biological imaging, 10 catalysis, and light-emitting devices. 6 Amongst the MNCs, those of gold (Au) and silver (Ag) have been extensively explored due to their higher stability, ease of synthesis, and biocompatibility. ...
... The retention of the absorption and emission intensity after one year with respect to the initial intensities were B98 and B91%, indicates stable CuNC@ N-GQDs. The temporal stability reported 7,25,40 for CuNCs are 21, 30 and 180 days and the highest hitherto is B180 days. 7 The comparatively higher stability of CuNC@N-GQDs is therefore attributed to the presence of N-GQDs. ...
... The temporal stability reported 7,25,40 for CuNCs are 21, 30 and 180 days and the highest hitherto is B180 days. 7 The comparatively higher stability of CuNC@N-GQDs is therefore attributed to the presence of N-GQDs. Even though the N-GQDs were added as a reducing agent, their presence in the product NC indicates that they act as a capping agent/ligand similar to GSH. ...
A highly stable copper (Cu) nanocluster (NC), which exhibited stability for more than one year, was synthesized using nitrogen-doped graphene quantum dots (N-GQDs) as reducing and capping agents and smaller glutathione molecules as additional capping agents. The synthesized NC, CuNC@N-GQDs, successfully sensed dopamine (DA), serotonin (SER), and nicotine (NIC) simultaneously with well-defined peaks and good peak-to-peak separation, whereas none of the controls including CuNCs and N-GQDs exhibited the simultaneous sensing properties. In addition, they exhibited enhanced sensitivity with current responses ∼4, ∼4, and ∼2 times those of the corresponding control for DA, SER, and NIC. The limits of detection obtained were 0.001, 1.00, and 0.01 nM for DA, SER, and NIC, respectively. The higher sensitivity and the simultaneous sensing are indicative of the synergistic effect of CuNCs and N-GQDs in the CuNC@N-GQDs. The sensing performance was successfully extended to real blood and urine samples spiked with DA, SER, and NIC.
... 18−23 Although the stability of the Cu NCs somewhat increased by using these methods, they showed rapid deactivation after 30 h of irradiation with UV light. 23,24 Nevertheless, the limited stability of the Cu NCs could allow studying the ultrafast electron and energy transfer dynam-ics. 21−25 We recently showed that L-cysteine-protected Cu NCs can act as both source and sink for electrons in the presence of a suitable quencher. ...
... 21−25 We recently showed that L-cysteine-protected Cu NCs can act as both source and sink for electrons in the presence of a suitable quencher. 22,23 Koninti et al. elaborated the fast dynamics of Cu NCs inside reverse micelles (RMs), 24 and Ou et al. studied PET between Cu NCs and G-quadruplex DNA. 25 The partially solved stability problem of the Cu NCs found light when Koninti et al. 24 adapted the concept of Lisiecki and Pileni 26 to synthesize Cu NCs inside the aqueous pool of RMs. They found that Cu NCs emit from the metalcore and surface states. ...
... 21−25 We recently showed that L-cysteine-protected Cu NCs can act as both source and sink for electrons in the presence of a suitable quencher. 22,23 Koninti et al. elaborated the fast dynamics of Cu NCs inside reverse micelles (RMs), 24 and Ou et al. studied PET between Cu NCs and G-quadruplex DNA. 25 The partially solved stability problem of the Cu NCs found light when Koninti et al. 24 adapted the concept of Lisiecki and Pileni 26 to synthesize Cu NCs inside the aqueous pool of RMs. They found that Cu NCs emit from the metalcore and surface states. ...
The perennial problem of instability of fluorescent copper nanoclusters (Cu NCs), stemming principally from aerial oxidation, has prevented their vivid usage in energy harvesting compared to the other metal NCs. However, replacement of the much expensive metal NCs with the cheaper Cu NCs is desirable if the functions are met with. Although thiolate protection of Cu NCs could bring some stability to them, appreciably decentlystable Cu NCs were produced inside the aqueous core of reverse micelles (RMs). However, this recent development has not been further explored on the photosensitization of the Cu NCs inside the RMs and their controlled modulation as energy antenna. Here we have synthesized stable Cu NCs inside the aqueous core of RMs with three different pool sizes and established photoinduced electron transfer (PET) to an electron acceptor. Considering the bulk quencher concentration, it appears that the extent of PET increases with decrease in the size of the aqueous core of RMs. However, calculating the effective concentration of the electron acceptor inside the RMs and considering the polarity of the microheterogeneous systems, it becomes clear that the extent of PET actually decreases with decrease in the size of the aqueous pool (w0, i.e., [H2O]/[AOT]) = 5-20) in the RMs. This proof of concept and the results are promising toward applications in PET-driven phenomena such as solar cells or batteries.
... Lifetimes in several hundreds of picosecondst os everal nanoseconds timescales could be attributed to the fluorescence lifetimes of the molecular like states of the Cu NCs originating due to quantum confinement (Table 1). [32] The average lifetime (8.25 ns) of pure Cu NCs decreases in case of all the three complexes and the extent of lifetime shortening is maximum in case of the Rhdimer (3), which corroborates with the steady state fluorescence results. The results indicate possibility of PET. ...
... The deactivation processes include the ultra-fast (1.1 ps) decay component due to internal conversion from S n to the first singlet excited state S 1 as explained before by Zhou et al. [33] The slower component of 23.2 ps arises due to the redistribution of the CuÀCu bond lengths. [11,32] The slowest component of 1.47 ns denotes the radiative relaxation of the system from S 1 to the ground state (S 0 ). Interestingly, on formation of the Cu NC-cyclometalated complex, the ultrafast component of 1.1 ps decreased considerably with ad ramatic increase in the percentage contribution. ...
... Reduction in the values of τ 2 and τ 3 indicates PET followed by charge transfer in the excited state. [32] The picosecond upconversion dynamics and band gap determination could not explain the time scale for PET. Femtosecond transient absorption (TA) spectroscopy is thus usefuli nm onitoring the generation of the transient excited state species or the radical formation as ar esult of interaction between the Cu NCs and the cyclometalated complexes. ...
In recent times, copper nanocluster (Cu NC) has become a promising candidate as an interesting nanomaterial for its potential applications in optoelectronics, sensing, catalysis and bioimaging. Herein, we have synthesized L–Cysteine protected Cu NC in 1:1 water:acetonitrile (ACN) solvent possessing bright cyan emission (λem=495 nm) with 10.4% quantum yield. Furthermore, the cyclometalated complexes of Ir(III) and Rh(III) are light harvesters, which are applied in photoinduced electron and energy transfer and photocatalysis. Three such cyclometalated complexes of Rh and Ir have been employed herein, which on electrostatic interaction with Cu NCs in 1:1 water:ACN medium, quench the fluorescence from Cu NC dramatically. This quenching could be suitably attributed to photoinduced electron transfer (PET) where Cu NC acts as an electron acceptor. PET has been confirmed by steady state and time resolved fluorescence spectroscopy as well as by ultrafast femtosecond upconversion and transient absorption spectroscopy. Photoinduced electron transfer between cyclometalated Ir and Rh complexes and copper nanoclusters in mixed solvent can be quantified by ultrafast fluorescence spectroscopy.
... The FTIR spectra of L-Cys exhibited numerous characteristics bands such as -SH stretching at 2532 cm −1 which disappeared in the spectrum of the nanoclusters. This suggests the existence of V atoms from VNCs through the formation of the VeS bonds ( Fig. 1c) [41]. The XPS survey spectrum of the VNCs (Fig. S5), the oxidation state of V, and the binding energy position of V2p 3/2 and V2p 1/2 located at 517.2 eV and 523.9 eV [42], indicate that the VNCs were composed of V(0) or V(I), respectively (Fig. 1d). ...
Highly selective and rapid sensing of adenosine (AdN) based on novel photoluminescent vanadium nanoclusters (VNCs) anchored on molybdenum disulfide nanosheets (MoS2-NS) are reported for the first time. The remarkably photoluminescent VNCs features excellent water solubility, ultra-small size, and bright photoluminescence. The VNCs on the MoS2-NS (MoS2-NS/VNCs) are well dispersed in aqueous solution and features a remarkable increase in bandgap energy (2.67 eV) compared to MoS2-NS (2.10 eV). The photoluminescence of VNCs was quenched by MoS2-NS via photoluminescence resonance energy transfer (PRET) mechanism. Furthermore, MoS2-NS/VNCs exhibit high selectivity and sensitivity toward AdN over other AdN analogues, biomolecules, and metal ions. Consequently, the absorption spectral maxima appeared at 650 nm and emission significantly enhanced in intensity at 525 nm. Interestingly, the stronger binding of AdN with MoS2-NS/VNCs was fast with nearly 65.7 % photoluminescence restored in less than 5 min. The PL lifetime of MoS2-NS/VNCs (τ1 = 8.20 ns, τ2 = 0.68 ns) increased (τ1 = 10.48 ns, τ2 = 0.79 ns) in the presence of AdN. The observed limit of detection was 29 pM. This VNCs sensor system can be constructed as XNOR logic gate with MoS2 and AdN as chemical inputs by examining the mode of emission. Finally, this probe was used to detect AdN in live cells and human serum samples with satisfactory results.
... The FTIR spectra of L-Cys exhibited numerous characteristics bands such as -SH stretching at 2532 cm −1 which disappeared in the spectrum of the nanoclusters. This suggests the existence of V atoms from VNCs through the formation of the VeS bonds ( Fig. 1c) [41]. The XPS survey spectrum of the VNCs (Fig. S5), the oxidation state of V, and the binding energy position of V2p 3/2 and V2p 1/2 located at 517.2 eV and 523.9 eV [42], indicate that the VNCs were composed of V(0) or V(I), respectively (Fig. 1d). ...
Highly selective and rapid sensing of adenosine (AdN) based on novel photoluminescent vanadium nanoclusters (VNCs) anchored on molybdenum disulfide nanosheets (MoS2-NS) are reported for the first time. The remarkably photoluminescent VNCs features excellent water solubility, ultra-small size, and bright photoluminescence. The VNCs on the MoS2-NS (MoS2-NS/VNCs) are well dispersed in aqueous solution and features a remarkable increase in bandgap energy (2.67 eV) compared to MoS2-NS (2.10 eV). The photoluminescence of VNCs was quenched by MoS2-NS via photoluminescence resonance energy transfer (PRET) mechanism. Furthermore, MoS2-NS/VNCs exhibit high selectivity and sensitivity toward AdN over other AdN analogues, biomolecules, and metal ions. Consequently, the absorption spectral maxima appeared at 650 nm and emission significantly enhanced in intensity at 525 nm. Interestingly, the stronger binding of AdN with MoS2-NS/VNCs was fast with nearly 65.7% photoluminescence restored in less than 5 minutes. The PL lifetime of MoS2-NS/VNCs (τ1 = 8.20 ns, τ2 = 0.68 ns) increased (τ1 = 10.48 ns, τ2 = 0.79 ns) in the presence of AdN. The observed limit of detection was 29 pM. This VNCs sensor system can be constructed as XNOR logic gate with MoS2 and AdN as chemical inputs by examining the mode of emission. Finally, this probe was used to detect AdN in live cells and human serum samples with satisfactory results.
... Lopez-Quintela and coworkers 167 employed the water-in-oil micro-emulsion technique to prepare blue emitting Cu NCs under UV irradiation, using Cu 2+ as an aqueous solution, cyclohexane as an oily phase, NaBH 4 as a reducing agent and sodium dodecyl sulfate with isopentanol as a surfactant. Recently, Koninti et al. 168 synthesized Cu NCs inside the water pool of reverse micelles, which were made of sodium bis(2-ethyl-1-hexyl)-sulfosuccinate or Triton X-100 or CTAB. Quite remarkably, the stability of L-cysteine protected Cu NCs increased from 40 days in bulk water to 180 days in reverse micelles due to the protection of the as-synthesized clusters from aerial oxidation. ...
Copper nanoclusters (Cu NCs) have emerged as a valuable member of the family of ligand-protected few-atomic metal nanoparticles and show fascinating properties of color-controlled light emission, combined with the advantages of versatile solution-based chemical synthesis at low cost. Synthetic methods of Cu NCs using various types of functional ligands and scaffolds allow tuning their emission wavelength and improving their environmental stability. Depending on the method of preparation and the ligands used, Cu NCs have already been applied for a wide variety of applications in catalysis, sensing, bioimaging, theranostics, and optoelectronics. This review highlights the potential of Cu NCs and links synthetic procedures and functionalization with different ligands with their properties and applications.
... In a very recent report (2018), Raj Kumar Koninti et al. have introduced a new concept about the origin of fluorescence in the transition metal nanoclusters. [79] From the different experimental results, they investigated that the appearance of two overlapped bands has different origin in the Cu NCs. The intrinsic emission band at the lower wavelength (≤ 470 nm) is originated from the metal core, while the extrinsic broad emission band at higher wave length (≥ 470 nm) is originated from the numerous surface states. ...
Sustainable future extremely demands the innovative breakthroughs in science and technology today especially in the energy sector. Earth‐abundant resources can be explored and used to develop renewable and sustainable resources of energy to meet the ever increasing global energy demand. The efficient solar‐powered conversion systems with the exploitation of cheap and robust catalytic materials for the photo and photo‐electro‐induced water splitting, photovoltaic cell, fuel cell and renovation of waste products (such as CO2) as chemical fuel are appealing solutions in this quest. Many electrocatalysts and nanomaterials have been extensively studied in this regard. Low overpotential, catalytic stability, and accessibility remain major challenges. Metal nanoclusters (NCs, ≤ 3 nm) with dimensions between molecule and nanoparticles (NPs) are innovative materials in catalysis science. They behave like a super atom with exciting size and facet‐dependent properties and dynamic intrinsic characteristic. Being an emerging field in recent scientific endeavors, the metal NCs are believed to come up to copycat the natural photosystem (II) for the generation of green electrons in a viable way to facilitate the challenging catalytic processes in energy conversion schemes. This review aims to discuss the metal NCs in terms of their unique physicochemical properties, possible synthetic approaches by wet‐chemistry, and various applications mostly the recent advancements in electrochemical, photo‐electrochemical water splitting cycle and oxygen reduction reaction (ORR) in fuel cell. Moreover, the significant role of MNCs in dye sensitized solar cells (DSSCs) and nanoarrays as a light harvesting antenna, electrochemical reduction of CO2 into fuel and concluding remarks about the present and future perspective of MNCs in the frontiers of surface science are also critically reviewed.
Methyl parathion (MP) residues have aroused extensive attention on account of their significant threat to the environment and food safety. Currently reported fluorescent methods used for MP sensing largely depend upon an enzyme. Designing a facile and specific enzyme-free MP fluorescent sensor is in great demand, which remains a challenge. Here, negatively charged Cu nanoclusters (CuNCs) anchored on positively charged melamine-formaldehyde (MF) microspheres (MF@CuNCs) through an electrostatic interaction were prepared. MF microspheres triggered aggregation-induced emission (AIE) of CuNCs and successfully circumvented the shortcomings of poor stability and low luminescence of CuNCs. The fluorescence intensity of MF@CuNCs can be quenched by p-nitrophenol produced by MP under alkaline conditions. Accordingly, a specific enzyme-free MP sensing method was constructed with MF@CuNCs. In combination with a smartphone, visually quantitative analysis of MP in a fast and portable way was also achieved. For the first time, AIE of CuNCs used for enzyme-free MP sensing was successfully explored in this work, and it is believed that this method will open a new pathway for AIE of CuNCs to be applied in various applications.
In recent years, fluorescence assemblies based amphiphilic molecules have gained attention as unique and powerful materials for multiple applications that cover sensors, optoelectronics and bioimaging because amphiphilic molecules self-assembly with outstanding flexibility and diversity spanning assembly structure from micelles, vesicles and nano-assemblies to gels. Weak and noncovalent interactions are important driving force for assemblies. The combination of the structural characteristics of self-assembly and the fluorescent properties of the fluorescent building element render the fluorescent material versatility and their easy-to-tune properties. Amphiphilic molecules can be used as building elements to co-assemble with dye molecules, aggregation-induced emission (AIE) gens, fluorescent nanoparticles and new amphiphilic molecules containing fluorescent groups can also be designed and prepared with self-assembly capability. Concomitantly, the improvement of fluorescence performance including fluorescence intensity, quantum yield, stability and controllability during assembly proved outstanding properties of fluorescence assemblies. These promising fluorescence assemblies are by far not exhaustive in construction method and mechanism explanation but foreshadow their more potential applications. Here, we will understand deeper the fluorescent assemblies and inspire future developments and applications employing this emerging fluorescence soft materials.
An emerging strategy for synthesizing nanoclusters and nanoparticles involves the confinement of particle precursors within small volumes and the subsequent reduction and aggregation of those precursors into discrete particles. These spatially isolated volumes are termed nanoreactors, and they impose barriers that not only restrict the movement of metal atoms and other reactants but also provide reaction conditions that are distinct from those of the surrounding environment. Nanoreactors for particle syntheses can be prepared by various strategies, which fall generally into two categories: solution-based and substrate-confined. Solution-based nanoreactors are broadly defined as 3D capsules that can be manipulated in solution, whereas substrate-confined nanoreactors are isolated volumes on a macroscopic substrate or surface. Here, we survey and analyse the merits of different nanoreactor techniques used to synthesize clusters and nanoparticles that cannot easily be made using traditional methods. We look at how the focus in this field has expanded beyond pure synthesis to making massive and complex libraries of materials and enabling exploration of the materials genome through high-throughput screening techniques.
A novel NIR fluorescent probe based on PVP–Au/CuNCs has been developed, exhibiting good selectivity and stability for detecting malachite green (MG).
Enhancement of the emission quantum yield and expansion of the emission tunability spectrum are the key aspects of an emitter, which direct the evolution of future generation light harvesting materials. In this regard, small molecular ligand-protected Cu nanoclusters (SLCuNCs) have emerged as prospective candidates. Herein, we report the broadband emission tunability in a SLCuNC system, mediated by in situ ligand replacement. 1,6-Hexanedithiol-protected blue emissive discrete Cu nanoclusters (CuNCs) and red emissive CuNC assemblies have been synthesized in one pot. The red emissive CuNC assemblies were characterized and found to be covalently-linked nanocluster superstructures. The blue emissive CuNC was further converted to a green-yellow emissive CuNC over time by a ligand replacement process, which was mediated by the oxidized form of the reducing agent used for synthesizing the blue emissive nanocluster. Steady-state emission results and fluorescence dynamics studies were used to elucidate that the ligand replacement process not only modulates the emission color but also alters the nature of emission from metal-centered intrinsic to ligand-centered extrinsic emission. Moreover, time-dependent blue to green-yellow emission tunability was demonstrated under optimized reaction conditions.
The fundamental challenges in application of copper nanoclusters (CuNCs) to fluorescence sensing are their poor stability, weak fluorescence intensity, and time-consuming preparation. In this work, an efficient and green method was developed by using a positively charged natural polysaccharide, chitosan (CS), as confining agent to electrostatically trap the negatively charged glutathione (GSH) as reducing-cum-protecting ligand for the in-situ synthesis of CuNCs. The synthesis can be completed within 1 min and CS-GSH-CuNCs emits intense orange fluorescence with an emission peak at 574 nm, which is much higher than that of GSH-CuNCs prepared in the absence of CS. Meanwhile, owning to both confinement and antioxidant effects of CS, CS-GSH-CuNCs shows excellent stability against time, temperature, salt, UV irradiation, and pH. Furthermore, the emission intensity of CS-GSH-CuNCs can be quenched by lysozyme (Lys) through the enzymatic hydrolysis of CS by Lys. Based on that, a fluorescence method was developed for the Lys determination with a linear range of 5–110 nM and a detection limit of 1.6 nM. Moreover, this approach shows high selectivity to Lys over various common substances in the biological environment, and thus can be used to determine Lys in human serum.
In the present work, we report a novel chemical approach for the synthesis of orange-red emitting copper nanoclusters (Cu NCs) using L-methionine as stabilizing agent at room temperature for the first time. The synthetic route is facile, economical and viable. The methionine stabilized copper nanoclusters (Cu NCs/Met) were thoroughly characterized by TEM, FT-IR, XPS, UV–Vis, steady state and transient fluorescence spectroscopy. The results show the synthesized Cu NCs/Met with a fluorescence quantum yield of 4.37% possessed high stability and excellent optical features such as large Stokes shift and long fluorescence lifetime (8.3 μs). Significantly, the fluorescence intensity of Cu NCs/Met could be efficiently quenched by norfloxacin (NOR) pharmaceutical. A fast and cost-effective NOR sensor was proposed employing Cu NCs/Met as the fluorescent nanoprobe, and the quenching mechanisms were attributed to inner filter effect and agglomeration-induced quenching. The developed sensor exhibited a high sensitivity and selectivity towards NOR in a wide linear range from 0.05 to 250 μM with a detection limit as low as 17 nM. Moreover, the practicability of the developed NOR sensor for real sample assay was validated with satisfactory recoveries, indicating this sensing platform with great potential for label-free pharmaceutical detection in complex systems.
Thermotropic liquid crystals (TLCs) are a series of soft materials with ordered arrangements of repeating structural units in a certain range of temperature. Specifically, the TLCs constructed by electrostatic combination between ionic surfactants and other oppositely charged groups have received great attention in the field of optoelectronic devices, material transportation and separation, smart materials as well as biosensing. As typical amphiphilic molecules, ionic surfactants with charged head-groups and long alkyl tails exhibit unique self-assembly behaviors, being preferable to act as flexible building blocks of TLCs. In this review, we introduced the TLCs formed by salt-free catanionic surfactants and ionic surfactants with other functional groups, including metal ions, nanoclusters, biomolecules and polyelectrolytes. The design of such TLC molecules, the influencing factors of mesophases and the related applications of the TLCs are discussed in detail. Because of the easy preparation and variable structure, these TLCs are hopeful to greatly widen the application scopes of the relevant functional molecules. At the end of this paper, we put forward some ideas in regard to the aspects requiring to be improved and propose the possible development trend for this direction. It is expected that through this review, the important roles of ionic surfactants in building TLCs will be highlighted, and thus more studies will be inspired to exploit more TLCs based on ionic surfactants with excellent properties.
Copper nanoclusters have showed tremendous application perspective due to their outstanding luminescence properties, inherent biocompatibility and easy accessibility; however, low emission brightness and poor resistance to ambient oxygen greatly limit their wide applications in long-term sensing and bioimaging. Herein, we develop an effective and general approach to enhance the luminescence and stability of copper nanoclusters through in situ generation of silica shell, and further propose a versatile surface functionalization method to achieve functional copper nanoclusters via the formation of second layer of silica shell. The use of thiol-containing trialkoxysilane facilitates the in situ generation of silica shell on the surface of copper nanocluster, and the existence of the first layer of silica shell substantially improves the emission efficacy of copper nanoclusters. The introduction of second layer of silica shell not only greatly enhances the brightness and stability of copper nanoclusters but also provides versatile surface functional groups via hydrolysis and polymerization of trialkoxysilance derivatives with different functionals on the surface of silica shell. Thiol-functionalized silica-coated copper nanoclusters as a demonstration are used to develop a novel detection strategy for hydrogen sulfide based on disulfide exchange reactions. Excellent analytical performance of the established method proves the feasibility and practicability of functional copper nanoclusters with core-shell structure in long-term sensing and bioimaging applications.
To address the concern of material chemists over the issue of stability and photoluminescent (PL) characteristics of Cu Nanoclusters (NCs), herein we present an efficient protocol discussing PL Cu NCs (Cu:HSA) having blue emission and high photo-stability. These PL NCs were illustrated as efficient probes for Förster Resonance Energy Transfer (FRET) with a compatible fluorophore (Coumarin 153). Our spectroscopic results were well complemented by our Molecular Docking calculations which also favored our proposed mechanism for Cu NC formation. The beneficial aspect and uniqueness of these non-toxic Cu:HSA NCs highlights its temperature dependent PL reversibility as well as the reversible FRET with Coumarin 153 which enables them to be used as a nano-thermometer and a PL marker for sensitive biological samples.
We present here a detailed time-dependent density-functional investigation aimed at systematically dissecting the electronic spectra of two thiolate and phosphine protected undecagold nanoclusters. Calculations performed on the experimental structures of Au11(PPh3)7Cl3 and Au11(PPh3)7(SPyr)3 show that ligands give negligible contributions in the visible region. Metal→ligand charge transfer transitions appear at energies well above the visible threshold, while transitions with some small ligand→metal and ligand→ligand character occur sporadically at even higher energies. Thus, the conjugation effect between the π-electrons of the ligand and electrons of gold, recently hypothesized to interpret the spectra of phosphine and thiolate-protected nanoclusters, is not confirmed by the results of this study.
The exact interaction between Au cores and surface ligands remains largely unknown because of the complexity of the structure and chemistry of ligand/Au-core interfaces in ligand-protected Au nanoclusters (AuNCs), which are commonly found in many organic-inorganic complexes. Here, femtosecond transient absorption measurement of the excited-state dynamics of a newly synthesized phosphine-protected cluster [Au20(PPhpy2)10Cl4]Cl2 () is reported. Intramolecular charge transfer (ICT) from the Au core to the peripheral ligands was identified. Furthermore, we found that solvation strongly affected ICT at ligand/Au-core interfaces while by choosing several typical alcoholic solvents with different intrinsic solvation times, we successfully observed that excited-state relaxation dynamics together with displacive excited coherent oscillation of Au20 clusters were significantly modulated through the competition between solvation and surface trapping. The results provide a fundamental understanding of the structure-property relationships of the solvation-dependent core-shell interaction of AuNCs for the potential applications in catalysis, sensing and nanoelectronics.
Highly luminescent and stable Cu nanoclusters (NCs) have been prepared, displaying an intriguing aggregation-induced emission (AIE) feature. The attractive AIE feature allowed the CuNCs to serve as pH stimuli-responsive functional materials. Additionally, we explored the utility of CuNCs for biosensing and catalysis applications.
Fluorescent clusters of several Ag atoms are fluorophores with a large set of attractive properties including sub-nanometer size, high fluorescence quantum yield and large Stokes shift. When formed on polymeric scaffolds they exhibit a broad range of heterogeneities in excitation and emission spectra. Addressing this issue we provide detailed spectroscopic studies of these clusters produced by photochemical reduction of silver salt on amphiphilic polystyrene-block-poly(methacrylic acid) block copolymer in N,N-dimethylformamide. Our results allow discriminating three types of emitters differing in positions of their excitation and emission bands. In addition, each band exhibits characteristic features of inhomogeneous broadening, such as strong dependences of excitation spectra on emission wavelength and of emission spectra on excitation wavelength in a mirror-image manner. The latter dependence does not appear on excitation at the 2nd band in the near-UV region. A qualitative model based on inhomogeneous broadening of spectra is suggested to explain these effects.
Ultra-small gold nanoclusters (Au NCs) are highly promising materials for tumor imaging and therapy because of their low toxicity, intrinsic fluorescence, and the availability of multifunctional groups for covalent linkage of diverse bioactive molecules. Au NCs stabilized by bovine serum albumin (BSA) were prepared via an improved "green" synthetic routine. To ameliorate the selective affinity of Au NCs for high folate receptor (FR) expressing tumors, folic acid (FA) was immobilized on the surface of Au NCs. Subsequently, a near-infrared (NIR) fluorescent dye MPA was conjugated with Au-FA NCs for in vitro and in vivo fluorescence imaging. Similarly, Doxorubicin (DOX), a widely used clinical anticancer drug, was also conjugated to the folate-modified Au NCs to form a prodrug (Au-FA-DOX). Cellular and in vivo acute toxicity studies demonstrated the low toxicity of the Au-FA-MPA to normal cells and tissues. Additionally, in vitro and in vivo study of the dynamic behavior and targeting ability of Au-FA-MPA to different tumors validated the high selective affinity of Au-FA-MPA to FR positive tumors. With regard to the Au-FA-DOX, high anti-tumor activity was displayed by this pro-drug due to the FR mediated uptake. Herein, all of the results supported the potential of using ligand-modified Au NCs for tumor imaging and targeted therapy.
A simple, low-cost and label-free Hg(2+) ion sensor has been developed by using novel luminescent Ag(+) nanoclusters (NCs) with an excellent optical property (quantum yield = 15%), an ultra-high ratio of active Ag(+) species in the NC surface (~100%), and an ultra-short diffusion path length of Hg(2+) ions to access the NC surface (~0.5 nm).
Highly fluorescent, water-soluble, few-atom noble-metal quantum dots have been created that behave as multielectron artificial atoms with discrete, size-tunable electronic transitions throughout the visible and near infrared. These molecular metals exhibit highly polarizable transitions and scale in size according to the simple relation E(Fermi)/N(1/3), predicted by the free-electron model of metallic behavior. This simple scaling indicates that fluorescence arises from intraband transitions of free electrons, and these conduction-electron transitions are the low-number limit of the plasmon-the collective dipole oscillations occurring when a continuous density of states is reached. Providing the missing link between atomic and nanoparticle behavior in noble metals, these emissive, water-soluble Au nanoclusters open new opportunities for biological labels, energy-transfer pairs, and light-emitting sources in nanoscale optoelectronics.
We report design strategies for the preparation of highly luminescent Au22(SG)18 clusters, where SG is glutathione, by the functionalization of the cluster shell. In these strategies, the cluster shell was covalently modified with small aromatic molecules and pyrene chromophores that led to a 5-fold PL enhancement by rigidifying the shell-gold. Highly luminescent water-soluble gold clusters with a PL quantum yield of 30% were obtained at room temperature. To further enhance the luminescence, the pyrene chromophores in the functionalized Au22 clusters were photoexcited at 350 nm to induce energy transfer from pyrene to the Au22 cluster. Steady-state and time-resolved PL measurements have shown evidence of enhanced rigidity with increased PL lifetimes for the functionalized Au22 clusters. However, the energy transfer efficiency was found to be only 14% because of the competing electron transfer deactivation pathway as evidenced by the formation of the pyrene anion radical revealed in the ultrafast transient absorption measurements. To suppress the electron transfer pathway, the pyrene functionalized Au22 clusters were ion-paired with tetraoctylammonium (TOA) cations that could break the electron transfer pathway, leading to a dramatic 37-fold increase in PL brightness with the resonance energy transfer efficiency of ca. 80%. This work presents effective design strategies for the preparation of highly luminescent gold clusters by the combination of rigidifying effect and energy transfer sensitization.
The pursuit of exceptionally high photoluminescence (PL) and stability is critical in the development of novel fluorophores for use in challenging bioimaging and optoelectronic devices. Carbon nanodots (CDs) doped with heteroatoms provide a particularly attractive means of effectively tailoring their intrinsic properties and exploiting new phenomena. Here, we report a one-step, scalable synthesis of boron-and-nitrogen-doped CD (BN-CD) with outstanding optical properties unlike those of nitrogen-doped CD (N-CD) in solid state as well as solution. The detailed mechanistic framework was explored using a series of spectroscopic analyses and ultrafast spectroscopy coupled with density functional theory calculations, which all conclusively confirmed that the presence of more graphitic structures in the core and well-distributed surface states are responsible for the enhanced PL in BN-CD. Furthermore, single-molecule spectroscopy analysis demonstrated that a single BN-CD show higher PL intensities and enhanced photobleaching time. We anticipate that this study will aid in uncovering the full potential of CDs in various fields.
Steady-state and time-resolved fluorescence spectroscopy techniques were used to probe multifluorescence resulting from citric-acid-derived carbon dots (C-dots). Commonly, both carboxyl-/amine-functionalized C-dots exhibit three distinct emissive states corresponding to the carbon-core and surface domain. The shorter-wavelength fluorescence (below 400 nm) originates from the carbon-core absorption band at ∼290 nm, whereas the fluorescence (above 400 nm) is caused by two surface states at ∼350 and 385 nm. In addition to three emissive states, a molecular state was also found in amine-functionalized C-dots. Time-resolved emission spectra (TRES) and time-resolved area normalized emission spectra (TRANES) were analyzed to confirm the origin of excitation wavelength-dependent fluorescence of C-dots. The surface functional groups on the C-dots are capable of regulating the electron transfer to affect the multifluorescence behavior. The electron transfer takes place from the carbon-core to surface domain by the presence of −COOH on the surface and vice versa for the case of −NH2 present on the surface. To the best of our knowledge, this is the first report that the multiemissive states are probed in C-dots systems using TRES and TRANES analyses, and related fluorescence mechanisms are verified clearly.
Metal nanoclusters (NCs) as a new class of phosphors have attracted a great deal of interests owing to their unique electronic structure and subsequently molecule-like optical properties. However, limited successes have been achieved in producing the NCs with excellent luminescent performance. In this paper, we demonstrate the significant luminescence intensity enhancement of 1-dodecanethiol (DT)-capped Cu NCs via self-assembly strategy. By forming compact and ordered assemblies, the original non-luminescent Cu NCs exhibit strong emission. The flexibility of self-assembly allows to further control the polymorphism of Cu NCs assemblies, and hence the emission properties. Comparative structural and optical analysis of the polymorphic NCs assemblies permits to establish a relationship between the compactness of assemblies and the emission. First, high compactness reinforces the cuprophilic Cu(I)•••Cu(I) interaction of inter- and intra-NCs, and meanwhile, suppresses intra-molecular vibration and rotation of the capping ligand of DT, thus enhancing the emission intensity of Cu NCs. Second, as to the emission energy that depends on the distance of Cu(I)•••Cu(I), the improved compactness increases average Cu(I)•••Cu(I) distance by inducing additional inter-NCs cuprophilic interaction, and therewith leads to the blue shift of NCs emission. Attributing to the assembly-mediated structural polymorphism, the NCs assemblies exhibit distinct mechanochromic and thermochromic luminescent properties. Metal NCs-based white light-emitting diodes are further fabricated by employing the NCs assemblies with blue-green, yellow, and red emissions as phosphors.
Luminescent nanomaterials have captured the imagination of scientists for a long time and offer great promise for applications in organic/inorganic light emitting displays, opto-electronics, optical sensors, biomedical imaging and diagnostics. Atomically precise gold clusters with well-defined core-shell structures present bright prospects to achieve high photoluminescence efficiencies. In this study, gold clusters with luminescence quantum yield higher than 60% were synthesized based on Au22(SG)18 cluster, where SG is glutathione, by rigidifying its shell-gold with tetraoctylammonium (TOA) cations. Time-resolved and temperature-dependent optical measurements on Au22(SG)18 have shown the presence of high quantum yield visible luminescence below freezing indicating that shell rigidity enhances the luminescence quantum efficiency. To achieve high rigidity of shell-gold, Au22(SG)18 was bound to bulky TOA that resulted in greater than 60% quantum yield luminescence at room temperature. Optical measurements have confirmed that the rigidity of shell-gold was responsible for the luminescence enhancement. This work presents an effective strategy to enhance the photoluminescence efficiencies of gold clusters by rigidifying the Au(I)-thiolate shell.
We present the solution synthesis of thiolated 2-methacryloyloxyethyl phosphorylcholine (MPC)-protected Au nanoclusters (NCs). This water-soluble lipid-mimetic MPC has been first used for the size focusing synthesis of thiolate (SR)-protected Aun(SR)m NCs. Au25(MPC)18 and Au4(MPC)4 NCs are selectively synthesized, without the need for electrophoretic or chromatographic isolation of size mixed products, by including ethanol or not in the solvent. The Au4(MPC)4 NCs emit at yellow wavelengths (580–600 nm) with a quantum yield (3.6 %) and an average lifetime of 1.5 μs. Also for the first time, we report C-reactive protein (CRP) sensing using Au NCs, with a detection limit (5 nM) low enough for the clinical diagnosis of inflammation. This is based on the quenching effect of specific CRP–MPC interactions on the fluorescence of the Au4(MPC)4 NCs.
Atomically precise Au nanoclusters (NCs) have emerged as fascinating fluorescent nanomaterials and attracted considerable research interest in both fundamental research and practical applications. Due to enhanced quantum confinement, they possess extraordinary optical, electronic, and magnetic properties and therefore are very promising for a wide range of applications, including biosensing, bioimaging, catalysis, photonics, and molecular electronics. Remarkable progress has been reported for the fundamental understanding, synthesis techniques, and applications. In this review, the updated advances are summarized in Au NCs, including synthesis techniques, optical properties, and applications. In particular, we focus on the optical properties and electron dynamic processes. In addition, the progress in other noble metallic NCs is included in this Review, such as Ag, Cu, Pt, and alloy, which have attracted much research interest recently. Finally, an outlook is presented for such fascinating nanomaterials in both aspects of future fundamental research and potential applications.
The excited state proton transfer (ESPT) dynamics of a potentially important anti-cancer drug, Topotecan (TPT) has been explored in aqueous reverse micelle (RM) using steady-state and time-resolved fluorescence measurements. Both the time-resolved emission spectrum (TRES) and time-resolved area normalized emission spectrum (TRANES) infer the generation of excited state zwitterionic form (Z*) of TPT from the excited state cationic form (C*) of TPT, as a result of ESPT process from the -OH group of TPT to the nearby water molecule. The ESPT dynamics was found to be severely retarded inside the nano-cavities of RMs, yielding time constants of 250 ps to 1.0 ns, which is significantly slower than the dynamics obtained in bulk water (32 ps). The observed slow ESPT dynamics in RM compared to bulk water is mainly attributed to the sluggish hydrogen bonded network dynamics of water molecules inside the nano-cavity of RM and the screening of the sodium ions present at the interface.
Herein, an innovative and simple strategy for synthesizing high fluorescent Cu nanoclusters was successfully established while L-cysteine played a role as the stabilizer. Meaningfully, the current Cu nanoclusters together with a quantum yield of 14.3% were prepared in aqueous solution, indicating their extensive applications. Subsequently, the possible fluorescence mechanism was elucidated by fluorescence, UV-vis, HR-TEM, FTIR, XPS, and MS. Additionally, the CuNCs were employed for assaying Hg2+ on the basis of the interactions between Hg2+ and L-cysteine; thus facilitating the quenching of their fluorescence. The proposed analytical strategy permitted detections of Hg2+ in a linear range of 1.0 x 10(-7) mol L-1 x 10(-3) mol L-1, with a detection limit of 2.4 x 10(-8) mol L-1 at a signal-to-noise ratio of 3. Significantly, this CuNCs described here were further applied for coding and fluorescent staining, suggesting may broaden avenues toward diverse applications.
Fluorescent Au25 nanoclusters recently have drawn considerable research interest due to their unique properties and potential applications. Despite significant advances in their synthesis methods and application development, the origin of the fluorescence and underlying mechanism still remain unclear. In this work we investigate the fluorescence dynamics in BSA-protected Au25 nanoclusters by time-resolved photoluminescence and transient absorption techniques covering picosecond to microsecond time scales. We demonstrate here that the red fluorescence consists of both prompt fluorescence and thermally activated delayed fluorescence, and the latter is more dominant. A small energy gap of 30 meV between the triplet and the singlet states was determined from our temperature-dependent time-resolved fluorescence measurement. Moreover, we elucidate that the absorption band at 2.34 eV corresponds to the HOMO–LUMO transition in this system due to the interaction between Au25 NCs and BSA. We also show that an effective relaxation pathway exists from the higher excited state to the LUMO.
“Catching” metals in the nonmetallic form in solution, as they grow to bulk, is one of the most exciting areas of contemporary materials research. A new kind of stabilization to catch the nonmetallic form of noble metals with small thiols has evolved as an exciting area of synthesis during the past decade. Gold clusters stay in the frontline of this research, yielding new “molecules” composed of a few to several hundreds of atoms. By taking guidelines from gold cluster research, various new protocols for silver nanoclusters were developed. In this Perspective, we highlight the recent advances on the synthesis of atomically precise silver, gold, and their alloy clusters with a special emphasis on silver. As a result of intense efforts of the recent past, clusters such as Ag7,8(SR)7,8, Ag7(−S–R–S−)4, Ag9(SR)7, Ag32(SR)19, Ag44(SR)30, Ag140(SR)53, Ag280(SR)140, and Ag152(SR)60 (SR and S–R–S refer to thiolate and dithiolate ligands, respectively) were added to the literature. Moreover, “silver-covered” and “gold-covered” alloy clusters have also been synthesized. Early reports of the crystallization of such clusters are available. Several of these clusters are shown to act as sensors, catalysts, and pesticide degradation agents, which suggests that these materials may find applications in daily life in the foreseeable future.
We discuss a complete mechanistic study on the anisotropic growth of zinc oxalate nanostructures within reverse micelles. We have employed small angle X-ray scattering (SAXS), dynamic light scattering (DLS) and transmission electron microscopy (TEM) to understand the detailed growth of the nanostructures. We have been able to observe the generation of nuclei and their aggregation to a critical size beyond which they form nanostructures of higher dimensions in self assembled templates. One of our aim was to find a correlation between size and shape of microemulsion droplets (MDs) and that of the resulting nanostructures of zinc oxalate (ZO) which grow within the MDs. Combination of SAXS and DLS show in situ growth of nanoparticles in the individual droplets which consume the water insoluble product formed and undergo exchange coalescence with other droplets. The structural transition of the MDs is captured by observing the change in shape anisotropy, together with a detailed structural analysis of micelles in which the nanostructures grow as a function of time. Importantly, once the reaction is triggered, the nucleation of the droplets start instantly and a very short period is noticed where MDs become cylindrical with approximate aspect ratio of 4:1 in which nanostructures grow anisotropically and achieve an average critical size of 55 nm (elongated nanoparticles) signifying the existence of short nucleation-dominant particle growth period, beyond which a transition from elongated nanostructures to small rods is observed. The critical size for the elongated droplets is 80 nm in length and 18 nm in diameter and these critical dimensions at the point of transition are a new finding about an asymmetric particle before the rods begin to start self-assembling. Once the shape of microemulsions turns cylindrical the dynamical exchange with other microemulsions is very fast at both ends, resulting in the formation of nanorods of zinc oxalate and an increase in the aspect ratio of these rods. This growth process can be viewed as a morphologically templated nucleation process and the droplets act as shaping vesicles for the formation of ZO nanorods. This study is significant since it attempts to correlate the size and shape of the reverse micellar (microemulsion) droplets with the new born product nanoparticles inside the droplets and the subsequent growth of the nanoparticles within the droplets.
Fluorescent metal nanoclusters (NCs) are a class of emerging fluorescent materials. They have excellent photostability and biocompatibility with sub-nanometer size and are easy to synthesize. Taking advantage of these features, fluorescent metal NCs have been involved in exciting developments of analytical methods for fluorescent biosensing and bioimaging. In this review, we first summarize the approaches to synthesis and bioconjugation for fluorescent metal NCs (Ag, Au, Cu and Pt). We then highlight their applications as fluorescent probes for metal ions, small molecules, nucleic acids, and protein detection. We also summarize the use of metal NCs in cellular and in-vivo targeting and imaging. Finally, we envision the various prospects for research on fluorescent metal NCs in the future.
Highly fluorescent copper nanoclusters (Cu NCs) have been synthesised using single-step reduction of copper sulphate by hydrazine, in the presence of lysozyme. The fluorescence quantum yield was measured to be as high as 18%. The emission was also found to be dependent on the excitation wavelength. Mass spectrometric analyses indicated the presence of species corresponding to Cu2 to Cu9. Transmission electron microscopic analyses indicated the formation of agglomerated particles of average diameters of 2.3 nm, which were constituted of smaller particles of average diameter of 0.96 nm. They were found to be stable between pH 4-10 and in addition having excellent chemical and photostability. The non-cytotoxic NCs were used to successfully label cervical cancer HeLa cells.
Femtosecond fluorescence upconversion measurements are employed to elucidate the mechanism of ultrafast double proton transfer dynamics of BP(OH)2 inside molecular containers (cucurbit[7]uril (CB7) and β-cyclodextrin (β-CD)). Femtosecond up-converted signals of BP(OH)2 in water consist of growth followed by a long decay component (∼650 ps). The appearance of the growth component (∼35 ps) in the up-converted signal indicates the presence of a two-step sequential proton transfer process of BP(OH)2 in water. Surprisingly, the up-converted signal of BP(OH)2 inside the CB7 nano-cavity does not exhibit any growth component characteristic of a two-step sequential process. Interestingly, the growth component exists inside the nano-cavity of β-CD (having similar cavity size as that of CB7), inferring the presence of a two-step sequential process of PT inside the β-CD nano-cavity. The different features of PT dynamics of BP(OH)2 in the above mentioned two macrocyclic hosts may be attributed to the presence and absence of water solvation network surrounding the BP(OH)2 inside the nano-cavities of β-CD and CB7, respectively. Finally, docking and DFT calculations have been employed in deciphering the molecular pictures of the interactions between BP(OH)2 and the macrocyclic host.
Gold and silver nanoclusters or Au/Ag NCs with core sizes smaller than 2 nm have been an attractive frontier of nanoparticle research because of their unique physicochemical properties such as well-defined molecular structure, discrete electronic transitions, quantized charging, and strong luminescence. As a result of these unique properties, ultrasmall size, and good biocompatibility, Au/Ag NCs have great potential for a variety of biomedical applications, such as bioimaging, biosensing, antimicrobial agents, and cancer therapy. In this feature article, we will first discuss some critical biological considerations, such as biocompatibility and renal clearance, of Au/Ag NCs that are applied for biomedical applications, leading to some design criteria for functional Au/Ag NCs in the biological settings. According to these biological considerations, we will then survey some efficient synthetic strategies for the preparation of protein- and peptide-protected Au/Ag NCs with an emphasis on our recent contributions in this fast-growing field. In the last part, we will highlight some potential biomedical applications of these protein- and peptide-protected Au/Ag NCs. It is believed that with continued efforts to understand the interactions of biomolecule-protected Au/Ag NCs with the biological systems, scientists can largely realize the great potential of Au/Ag NCs for biomedical applications, which could finally pave their way towards clinical use.
Super-small Au8 nanoclusters have shown great potential to be used in bioimaging, biosensors and catalysis. Understanding the fluorescence origin and the spectral broadening mechanism is of critical importance for the applications. Here we investigate the fluorescence origin and the spectral broadening mechanism using steady state and ultrafast time-resolved spectroscopy. For the first time we clearly elucidate the broad fluorescence of Au8 nanoclusters consisting of an intrinsic band from the Au8 core and an extrinsic band from the surface fluorophores. The emission energy of the intrinsic band is in accord with the rule of E/N(1/3) and the spectral broadening originates from ultrafast dephasing due to effective electron-electron scattering. The extrinsic band has a much larger bandwidth due to massive surface fluorophores; it is the dominant mechanism for spectral broadening in Au8. In contrast to the jellium model predictions, the overall fluorescence exhibits excitation wavelength dependence.
Fitting (nanoparticles) to a T: Single-stranded DNA poly T (blue, see scheme) was found to be a template for the formation of fluorescent copper nanoparticles (CuNPs, red spheres). The size and fluorescence of these CuNPs could be regulated by altering the length of poly T. Other ssDNAs (green) failed to act as a template for CuNPs and were used to create alternating metallized and non-metallized nanostructures.
A facile and versatile method for preparing water-soluble, stable, luminescent Cu nanoclusters (NCs) via the process of size-focusing etching from nonluminescent nanocrystals is presented. Using glutathione as a model ligand, the smallest cluster, Cu2 , is selectively synthesized to form a nearly monodisperse product, eliminating the need for tedious size fractionation. Evolution of photoluminescence and absorption spectra reveal that the formation of stable cluster species occurs through surface etching. Intriguingly, the as-prepared CuNCs exhibit an aggregation-induced emission enhancement effect. The CuNCs emit a faint light when dispersed in aqueous solution, but generate a striking fluorescence intensity enhancement upon aggregation. Armed with these attractive properties, the emissive CuNCs are expected to open new opportunities for the construction of light-emitting diodes, chemosensors, and bioimaging systems.
In the past 20 years, researchers studying nanomaterials have uncovered many new and interesting properties not found in bulk materials. Extensive research has focused on metal nanoparticles (>3 nm) because of their potential applications, such as in molecular electronics, image markers, and catalysts. In particular, the discovery of metal nanoclusters (<3nm) has greatly expanded the horizon of nanomaterial research. These nanosystems exhibit molecular-like characteristics as their size approaches the Fermi-wavelength of an electron. The relationships between size and physical properties for nanomaterials are intriguing, because for metal nanosystems in this size regime both size and shape determine electronic properties. Remarkably, changes in the optical properties of nanomaterials have provided tremendous insight into the electronic structure of nanoclusters. The success of synthesizing monolayer protected clusters (MPCs) in the condensed phase has allowed scientists to probe the metal core directly. Au MPCs have become the “gold” standard in nanocluster science, thanks to the rigorous structural characterization already accomplished. The use of ultrafast laser spectroscopy on MPCs in solution provides the benefit of directly studying the chemical dynamics of metal nanoclusters (core), and their nonlinear optical properties.
Many industrial catalysts involve nanoscale metal particles (typically 1–100 nm), and understanding their behavior at the molecular level is a major goal in heterogeneous catalyst research. However, conventional nanocatalysts have a nonuniform particle size distribution, while catalytic activity of nanoparticles is size dependent. This makes it difficult to relate the observed catalytic performance, which represents the average of all particle sizes, to the structure and intrinsic properties of individual catalyst particles. To overcome this obstacle, catalysts with well-defined particle size are highly desirable.
The luminescence of quantum-sized metal clusters has enthralled the scientific community in recent years. In this study, ultrafast luminescence dynamics of hexanethiol (C6S)- and glutathione (GS)-protected Au25 clusters are investigated with time-resolved luminescence spectroscopy. The focus of the present investigation is to understand the dynamics of higher excited states and also the relaxation of core Au states to Au semi-ring states in Au25L18 (“L” is the protecting ligand) clusters. Comparative luminescence measurements on larger monolayer-protected gold clusters (2.2 nm Au(C6S) and 2.2 nm Au(GS)) and gold nanoparticles (3 nm Au(C6S) and 13 nm Au(citrate-stabilized)) are also carried out. The investigated Au25L18 clusters have shown a low quantum-yield visible photoluminescence in addition to near-infrared luminescence, which is used as a probe to follow the dynamics of core Au states. The luminescence decay traces of Au25L18 clusters have shown unique ultrafast growth and decay kinetics that are absent in the larger monolayer-protected gold clusters. The growth time constants are independent of the passivating ligand, suggesting that the luminescence arises out of the Au25 core states. The decay of the luminescence is dependent on the passivating monolayer and is ascribed to the relaxation of the core Au states to S−Au−S−Au−S semi-ring states. However, the excited-state dynamics in Au25L18 clusters is not a typical two-state relaxation from core to semi-ring states, but rather proceeds through a manifold of electronic states as the luminescence traces show wavelength-dependent growth and decay kinetics. Also, femtosecond time-resolved luminescence measurements of Au25L18 have proved that the higher excited states in monolayer-protected Au25 clusters decay with a finite lifetime (200 fs up to a few picoseconds) that can be utilized for applications in solar energy harvesting and catalysis.
: A new template synthesis strategy for preparing Cu nanoclusters within dendrimer 'nanoreactors' is demonstrated. Hydroxyl-terminated polyamidoamine (PAMAM) dendrimers of generation higher than 2 act as monodispersed templates as well as stabilizers for nanocluster synthesis. Cu(2+) ions are first quantitatively sorbed into the dendrimer via a strong coordinative interaction with interior amines and then chemically reduced to yield Cu nanoclusters. The nanoclusters are composed of a well-defined number of atoms. Importantly, cluster size can be controlled by varying the size of the host dendrimer nanoreactor (16-atom Cu cluster in G4 and 64-atom Cu cluster in G6 dendrimers). The clusters remain trapped within the dendrimers for extended periods of time, do not agglomerate, and do not precipitate. The clusters can also be oxidized to yield dendrimer-encapsulated Cu(2+).
A novel, gold-nanocluster-based fluorescent sensor for cyanide in aqueous solution, which is based on the cyanide etching-induced fluorescence quenching of gold nanoclusters, is reported. In addition to offering high selectivity due to the unique Elsner reaction between cyanide and the gold atoms of gold nanoclusters, this facile, environmentally friendly and cost-effective method provides high sensitivity. With this sensor, the lowest concentration to quantify cyanide ions could be down to 200 × 10−9 M, which is approximately 14 times lower than the maximum level (2.7 × 10−6 M) of cyanide in drinking water permitted by the World Health Organization (WHO). Furthermore, several real water samples spiked with cyanide, including local groundwater, tap water, pond water, and lake water, are analyzed using the sensing system, and experimental results show that this fluorescent sensor exhibits excellent recoveries (over 93%). This gold-nanocluster-based fluorescent sensor could find applications in highly sensitive and selective detection of cyanide in food, soil, water, and biological samples.
Gold nanoparticles typically have a metallic core, and the electronic conduction band consists of quasicontinuous energy levels (i.e. spacing δ ≪ kBT, where kBT is the thermal energy at temperature T (typically room temperature) and kB is the Boltzmann constant). Electrons in the conduction band roam throughout the metal core, and light can collectively excite these electrons to give rise to plasmonic responses. This plasmon resonance accounts for the beautiful ruby-red color of colloidal gold first observed by Faraday back in 1857.
Metal nanoclusters have interesting steady state fluorescence emission, two-photon excited emission and ultrafast dynamics. A new subclass of fluorescent silver nanoclusters (Ag NCs) are NanoCluster Beacons. NanoCluster Beacons consist of a weakly emissive Ag NC templated on a single stranded DNA ("Ag NC on ssDNA") that becomes highly fluorescent when a DNA enhancer sequence is brought in proximity to the Ag NC by DNA base pairing ("Ag NC on dsDNA"). Steady state fluorescence was observed at 540 nm for both Ag NC on ssDNA and dsDNA; emission at 650 nm is observed for Ag NC on dsDNA. The emission at 550 nm is eight times weaker than that at 650 nm. Fluorescence up-conversion was used to study the dynamics of the emission. Bi-exponential fluorescence decay was recorded at 550 nm with lifetimes of 1 ps and 17 ps. The emission at 650 nm was not observed at the time scale investigated but has been reported to have a lifetime of 3.48 ns. Two-photon excited fluorescence was detected for Ag NC on dsDNA at 630 nm when excited at 800 nm. The two-photon absorption cross-section was calculated to be ∼3000 GM. Femtosecond transient absorption experiments were performed to investigate the excited state dynamics of DNA-Ag NC. An excited state unique to Ag NC on dsDNA was identified at ∼580 nm as an excited state bleach that related directly to the emission at 650 nm based on the excitation spectrum. Based on the optical results, a simple four level system is used to describe the emission mechanism for Ag NC on dsDNA.
In this study, we investigated the catalytic properties of N,N-dimethylformamide (DMF)-stabilized gold nanoclusters (AuNCs) in the reduction of 4-nitrophenol (PNP) to 4-aminophenol by NaBH(4), a well known model reaction to be catalyzed by metal surfaces. The DMF-stabilized AuNCs were prepared in DMF by a surfactant-free method. The DMF-stabilized AuNCs showed high catalytic activity even when used in small quantities (∼10(-7) g). The pseudo-first-order rate constant (k(app)) and activation energy were estimated to be 3 × 10(-3) s(-1) and 31 kJ mol(-1), respectively, with 1.0 μM of the gold catalyst at 298 K. The catalytic activity of the DMF-stabilized AuNCs was strongly influenced by the layer of adsorbed DMF on the Au NCs. This layer of adsorbed DMF prohibited the reactants from penetrating to the surface of the AuNCs via the diffusion at the beginning of the reaction, resulting in an induction time (t(0)) before PNP reduction began. Restructuring of the DMF layer (essentially a form of activation) was the key to achieving high catalytic activity. In addition, atomically monodisperse Au(25)(SG)(18)NCs (SG: glutathione) showed higher catalytic activity in the PNP reduction (k(app) = 8 × 10(-3) s(-1)) even with a low catalyst concentration (1.0 μM), and there was no induction time (t(0)) in spite of the strongly binding ligand glutathione. This suggested that the catalytically active surface sites of the Au(25)(SG)(18)NCs were not sterically hindered, possibly because of the unique core-shell-like structure of the NCs. Retaining these open sites on AuNCs may be the key to making the NCs effective catalysts.
Though creation and characterization of water soluble luminescent silver nanodots were achieved only in the past decade, a large variety of emitters in diverse scaffolds have been reported. Photophysical properties approach those of semiconductor quantum dots, but relatively small sizes are retained. Because of these properties, silver nanodots are finding ever-expanding roles as probes and biolabels. In this critical review we revisit the studies on silver nanodots in inert environments and in aqueous solutions. The recent advances detailing their chemical and physical properties of silver nanodots are highlighted with an effort to decipher the relations between their chemical/photophysical properties and their structures. The primary results about their biological applications are discussed here as well, especially relating to their chemical and photophysical behaviours in biological environments (216 references).
A one-pot synthesis of extremely stable, water-soluble Cu quantum clusters (QCs) capped with a model protein, bovine serum albumin (BSA), is reported. From matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectrometry, we assign the clusters to be composed of Cu(5) and Cu(13) cores. The QCs also show luminescence properties having excitation and emission maxima at 325 and 410 nm, respectively, with a quantum yield of 0.15, which are found to be different from that of protein alone in similar experimental conditions. The quenching of luminescence of the protein-capped Cu QCs in the presence of very low hydrogen peroxide concentration (approximately nanomolar, or less than part-per-billion) reflects the efficacy of the QCs as a potential sensing material in biological environments. Moreover, as-prepared Cu QCs can detect highly toxic Pb(2+) ions in water, even at the part-per-million level, without suffering any interference from other metal ions.
Double-strand DNA (dsDNA) can act as an efficient template for the formation of copper nanoparticles (Cu NPs) at low concentration of CuSO(4), and the formed Cu NPs have excellent fluorescence, whereas a single-strand DNA (ssDNA) template does not support Cu NPs' formation. This property of dsDNA-Cu NPs makes it suitable for DNA sensing. However, exploration of dsDNA-Cu NPs applied in biological analysis is still at an early stage. In this regard, we report herein for the first time a sensitive, cost-effective, and simple aptamer sensor (aptasensor) using dsDNA-Cu NPs as fluorescent probe. The design consists of a dsDNA with reporter DNA (here, aptamer) as template for the formation of Cu NPs, and the formed dsDNA-Cu NPs show high fluorescence. Using adenosine triphosphate (ATP) as a model analyte, the introduction of ATP triggers the structure switching of reporter DNA to form aptamer-ATP complex, causing the destruction of the double helix and thus no formation of the Cu NPs, resulting in low fluorescence. The preferable linear range (0.05-500 μM), sensitivity (LOD 28 nM), and simplicity for the detection of ATP indicate that dsDNA-Cu NPs may have great prospects in the field of biological analysis. We also use this novel fluorescent probe to determine ATP in 1% human serum and human adenocarcinoma HeLa cells. The dsDNA-Cu NPs probes provide recovery of 104-108% in 1% human serum and a prominent fluorescent signal is obtained in cellular ATP assay, revealing the practicality of using dsDNA-Cu NPs for the determination of ATP in real samples. Besides, this design is simply based on nucleic acid hybridization, so it can be generally applied to other aptamers for label-free detection of a broad range of analytes. Successful detection of cocaine with detection limit of 0.1 μM demonstrates its potential to be a general method.
Subnanometer-sized copper nanoclusters were prepared by a one-pot procedure based on wet chemical reduction. The structural characteristics of the 2-mercapto-5-n-propylpyrimidine-protected nanoclusters, Cu(n) (n ≤ 8), were determined by mass spectrometry. The Cu nanoclusters displayed apparent luminescence, with dual emissions at 425 and 593 nm, with quantum yields of 3.5 and 0.9%, respectively, and high electrocatalytic activity in the electoreduction of oxygen.
In this paper, we explore the possibility of using ultrasmall near-infrared (NIR) gold nanoclusters (AuNCs) as novel contrast imaging agents for tumor fluorescence imaging in vivo. The fluorescence imaging signal of the tail vein administrated AuNCs in living organisms can spectrally be well distinguished from the background with maximum emission wavelength at about 710 nm, and the high photostability of AuNCs promises continuous imaging in vivo. The uptake of AuNCs by the reticuloendothelial system is relatively low in comparison with other nanoparticle-based contrast imaging agents due to their ultrasmall hydrodynamic size (∼2.7 nm). Through the body weight change analysis, the results show that the body weight of the mice administrated with AuNCs has not been changed obviously in comparison with that of the control mice injected with PBS. Furthermore, using MDA-MB-45 and Hela tumor xenograft models, in vivo and ex vivo imaging studies show that the ultrasmall NIR AuNCs are able to be highly accumulated in the tumor areas, thanks to the enhanced permeability and retention (EPR) effects. And the tumor-to-background ratio is about 15 for 6 h postinjection. The results indicate that the ultrasmall NIR AuNCs appear as very promising contrast imaging agents for in vivo fluorescence tumor imaging.
chemical equation presented A helping strand: A method to selectively form copper nanoparticles in solution using double-stranded DNA has been developed. The size of the nanoparticles is controlled by the length of the dsDNA template, and single-stranded DNA did not act as a template (see scheme). Single-stranded overhangs in dsDNA were used to prepare a nanostructure in which two metallized dsDNA segments were linked together by a nonmetallized rigid linker.
Microemulsion-based synthesis is found to be a versatile route to synthesize a variety of nanomaterials. The manipulation of various components involved in the formation of a microemulsion enables one to synthesize nanomaterials with varied size and shape. In this tutorial review several aspects of microemulsion based synthesis of nanocrystalline materials have been discussed which would be of interest to a cross-section of researchers working on colloids, physical chemistry, nanoscience and materials chemistry. The review focuses on the recent developments in the above area with current understanding on the various factors that control the structure and dynamics of microemulsions which can be effectively used to manipulate the size and shape of nanocrystalline materials.
This study focuses on understanding the growth and control of nanostructures using reverse micelles. It has been earlier realized that parameters like surfactant, cosurfactant, and aqueous content influence the size and shape of the nanostructures obtained using reverse micelles. However, a concerted effort to understand the role of these factors on the growth of a specific nanomaterial is missing. In this study we have focused on one nanomaterial (copper oxalate monohydrate) and determined how the above-mentioned factors control the size, shape, aspect ratio, and growth of these nanostructures. Our results show that cationic surfactants (CTAB, TTAB, and CPB) favor the formation of nanorods of copper oxalate. The aspect ratio of these rods could be controlled to obtain nanocubes (approximately 80-100 nm) and nanoparticles (approximately 8-10 nm) in the CTAB system using longer chain cosurfactants like 1-octanol and 1-decanol, respectively. Nanocubes of approximately 50-60 and approximately 60-80 nm were obtained using nonionic surfactants Triton X-100 and Tergitol, respectively. The size of the nanostructures could also be controlled by varying the molar ratio of water to surfactant (W0) by using a nonionic (Triton X-100)-based reverse micellar system. The study espouses the versatility of the microemulsion method to realize a variety of nanostructures of copper oxalate monohydrate. Our results will be of use in extending these ideas to other nanomaterials.
Synthesis of ultrasmall water-soluble fluorescent gold nanoclusters is reported. The clusters have a decent quantum yield, high colloidal stability, and can be readily conjugated with biological molecules. Specific staining of cells and nonspecific uptake by living cells is demonstrated.
A simple, one-pot, "green" synthetic route, based on the "biomineralization" capability of a common commercially available protein, bovine serum albumin (BSA), has been developed for the preparation of highly stable Au nanocrystals (NCs) with red emission and high quantum yield.
In the past decade, colloidal solutions have been assumed to be very efficient templates for controlling particle size and shape. A large number of groups have used reverse micelles to control the size of spherical nanoparticles. This makes it possible to determine the various parameters involved in such processes, and demonstrates that nanoparticles can be considered to be efficient nanoreactors. However, some discrepancies arise. There are few reports concerning the control of particle shape, and it is still rather difficult to determine the key parameters, such as the adsorption of salts and other molecules, and the synthesis procedure. Here, we discuss these controls of the size and shape of inorganic nanomaterials.
Single-stranded oligonucleotides stabilize highly fluorescent Ag nanoclusters, with emission colors tunable via DNA sequence. We utilized DNA microarrays to optimize these scaffold sequences for creating nearly spectrally pure Ag nanocluster fluorophores that are highly photostable and exhibit great buffer stability. Five different nanocluster emitters have been created with tunable emission from the blue to the near-IR and excellent photophysical properties. Ensemble and single molecule fluorescence studies show that oligonucleotide encapsulated Ag nanoclusters exhibit significantly greater photostability and higher emission rates than commonly used cyanine dyes.
Photostable Copper Nanoclusters: Compatible Fö rster Resonance Energy-Transfer Assays and a Nanothermometer
- M Xia
- A Ghosh
- R Sahoo
- A K Ghosh
- S S Paul
- A Chattopadhyay
- A Wei
- W Lu
- Y Chen
- W Chen
- S Ghosh
- S Das
- N K Anand
- U Mukherjee
- S Zhao
- M Sun
- L Crooks
M.; Xia, A. Ultrafast Relaxation Dynamics of Phosphine-Protected,
Rod-Shaped Au 20 Clusters: Interplay between Solvation and Surface
Trapping. Phys. Chem. Chem. Phys. 2014, 16, 18288−18293.
(22) Ghosh, R.; Sahoo, A. K.; Ghosh, S. S.; Paul, A.; Chattopadhyay,
A. Blue-Emitting Copper Nanoclusters Synthesized in the Presence of
Lysozyme as Candidates for Cell Labeling. ACS Appl. Mater. Interfaces
2014, 6, 3822−3828.
(23) Wei, W.; Lu, Y.; Chen, W.; Chen, S. One-Pot Synthesis,
Photoluminescence, and Electrocatalytic Properties of SubnanometerSized Copper Clusters. J. Am. Chem. Soc. 2011, 133, 2060−2063.
(24) Ghosh, S.; Das, N. K.; Anand, U.; Mukherjee, S. Photostable
Copper Nanoclusters: Compatible Fö rster Resonance Energy-Transfer
Assays and a Nanothermometer. J. Phys. Chem. Lett. 2015, 6, 1293−
1298.
(25) Zhao, M.; Sun, L.; Crooks, R. M. Preparation of Cu
Nanoclusters Within Dendrimer Templates. J. Am. Chem. Soc. 1998,
120, 4877−4878.
(26) Qing, Z.; He, X.; He, D.; Wang, K.; Xu, F.; Qing, T.; Yang, X.
Femtosecond to Nanosecond Dynamics of 2,2[prime or minute]bipyridine-3,3[prime or minute]-diol inside the Nano-Cavities of Molecular Containers
- J H Werner
- H.-C Yeh
- J Sharma
- A P Shreve
- J S Martinez
Werner, J. H.; Yeh, H.-C.; Sharma, J.; Shreve, A. P.; Martinez, J. S.;
et al. Bright Two-Photon Emission and Ultra-Fast Relaxation
Dynamics in a DNA-Templated Nanocluster Investigated by UltraFast Spectroscopy. Nanoscale 2012, 4, 4247−4254.
(39) Yang, X.; Feng, Y.; Zhu, S.; Luo, Y.; Zhuo, Y.; Dou, Y. One-Step
Synthesis and Applications of Fluorescent Cu Nanoclusters Stabilized
by l-Cysteine in Aqueous Solution. Anal. Chim. Acta 2014, 847, 49−
54.
(40) Koninti, R. K.; Gavvala, K.; Sengupta, A.; Hazra, P. Excited State
Proton Transfer Dynamics of Topotecan inside Biomimicking
Nanocavity. J. Phys. Chem. B 2015, 119, 2363−2371.
(41) Gavvala, K.; Sengupta, A.; Koninti, R. K.; Hazra, P.
Femtosecond to Nanosecond Dynamics of 2,2[prime or minute]bipyridine-3,3[prime or minute]-diol inside the Nano-Cavities of
Molecular Containers. Phys. Chem. Chem. Phys. 2014, 16, 933−939.
(42) Jia, X.; Li, J.; Wang, E. Cu Nanoclusters with Aggregation
Induced Emission Enhancement. Small 2013, 9, 3873−3879.
(43) Jia, X.; Yang, X.; Li, J.; Li, D.; Wang, E. Stable Cu Nanoclusters:
From an Aggregation-Induced Emission Mechanism to Biosensing and
Catalytic Applications. Chem. Commun. 2014, 50, 237−239.
(44) Wu, Z.; Liu, J.; Gao, Y.; Liu, H.; Li, T.; Zou, H.; Wang, Z.;
Zhang, K.; Wang, Y.; Zhang, H.; et al. Assembly-Induced Enhancement of Cu Nanoclusters Luminescence with Mechanochromic
Property. J. Am. Chem. Soc. 2015, 137, 12906−12913.
(45) Pyo, K.; Thanthirige, V. D.; Yoon, S. Y.; Ramakrishna, G.; Lee,
D. Enhanced Luminescence of Au 22 (SG) 18 Nanoclusters via Rational
Surface Engineering. Nanoscale 2016, 8, 20008−20016.
(46) Choi, Y.; Kang, B.; Lee, J.; Kim, S.; Kim, G. T.; Kang, H.; Lee, B.
- P Yu
- X Wen
- Y.-R Toh
- X Ma
- Tang
Yu, P.; Wen, X.; Toh, Y.-R.; Ma, X.; Tang, J. Fluorescent Metallic
Nanoclusters: Electron Dynamics, Structure, and Applications. Part.
Part. Syst. Charact. 2015, 32, 142−163.