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Futuristic Trends in Chemical, Material Sciences & Nano Technology
e-ISBN: 978-93-5747-825-0
IIP Series, Volume 3, Book 13, Part 1, Chapter 11
FORENSIC APPLICATIONS OF CARBON DOTS
Copyright © 2024 Authors Page | 135
FORENSIC APPLICATIONS OF CARBON DOTS
Abstract
Carbon dots have received a lot of
interest because to their outstanding
fluorescence capabilities, low cost of
production, and non-toxic qualities. This
analysis dives into current developments in
fields such as criminal justice, forensic
toxicology, and anti-counterfeiting methods.
Because of their color-tunable actions in
response to incident radiation, C-dot-based
combinations have proven particularly
beneficial for improving latent fingerprints,
providing superior contrast against different
backgrounds. As optical nano probes, these
dots demonstrate amazing sensitivity and
selectivity, allowing for the exact detection
of diverse substances such as biological
molecules, pharmaceuticals, weapons of
mass destruction, heavy metals, and
hazardous chemicals. C-dots may be
effortlessly incorporated into ink and
polymeric formulation due to their adaptive
structural and chemical properties, ushering
in a revolutionary era of inexpensive barcode
as well as nano tags for objects to be
identified and anti-counterfeit applications.
To assure significant societal and economic
benefits, the transition from these promising
research discoveries into effective advances
requires a coordinated strategy comprising
materials researchers, biologists, legal
professionals, and digital engineers.
Keywords: carbon dots; anti-counterfeiting;
molecular sensing; drugs; explosives;
fluorescence; fingerprinting.
Authors
Erra Adithi
Department of Forensic Science
Aditya Degree & PG. College
Surampalem, Andhra Pradesh, India
Vilas A. Chavan
Department of Forensic Science
Aditya Degree & PG. College
Surampalem, Andhra Pradesh, India
Ariya P
Department of Forensic Science
Aditya Degree & PG. College
Surampalem, Andhra Pradesh, India
BVSS Udaynadh
Department of Forensic Science
Aditya Degree & PG. College
Surampalem, Andhra Pradesh, India
Arpita Singh
Department of Cyber Forensics
Aditya Degree & PG. College
Surampalem, Andhra Pradesh, India
Futuristic Trends in Chemical, Material Sciences & Nano Technology
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FORENSIC APPLICATIONS OF CARBON DOTS
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I. INTRODUCTION
Carbon dots (C-dots) remain among the vanguard of advancing the science of
materials, chemical science, nanophysics, medical research, and engineering. Their influence
extends from fundamental research to use in practice. C-dots are extraordinarily photoactive
members of the Nano carbon family, with unique photophysical properties. Particularly, their
emission properties change with activation wavelength, and they exhibit increased photo
bleaching resistance. This unusual combination of qualities distinguishes C-dots and drives
innovation in a variety of industries [1–3]. Their elemental composition (often C, H, O, N,
with certain heteroatoms that are like S and P), functionalization of the surface, and
suspension environment all have a substantial impact on their yield of quantum information.
[4–6]. The quantum confinement impacts of graphene dots containing just a few monolayers
with tiny graphene within their centers are well defined. [7]. Simple oxidative reactions
modify the surfaces of C-dots, increasing their ability to move around in polar fluids and
ensuring long-term colloidal stability. Furthermore, the introduction of a modest
electrochemical field can affect the level of conjugation inside the carbogenic core and also
produce oxygenated defects. [8].
C-dots, sometimes known as the innocuous equivalents of quantum dots, are
manufactured at a low cost by pyrolysis or hydrothermal treatment of easily available natural
resources such as agro-waste and biomass. [9], grass [10], fruit juice [11], leaves [12],
glucose [13], gelatin [14], eggs [15], hair fibers [16], etc. Top-down procedures such as arc
discharge, ablation with lasers, oxidative, as well as electro-oxidative treatment of nanotubes
made of carbon can also be employed to produce well-defined C-dots. [17], carbon fibers
[18], activated carbon [19], exhaust soot [20], etc. In theory, these methods are scalable and
rely on simple synthetic methodologies followed by standard size exclusion along with
purification procedures like as centrifugation after filtering, and dialysis.
With relation to C-dots applications, special emphasis is made on developing
bioimaging nanoprobes with higher spatial resolution and accuracy. [21], nano-vehicles for
self-targeting drug delivery [22], photodynamic therapy agents [23], antimicrobial materials
[24], advanced sensors for chemical and biological compounds [25], technologies for water
and soil decontamination [26], slow-release fertilizers [27], polymer nanocomposites [28],
highly efficient photocatalysts [29] and superior energy convertors [30].
The present article will look at the real-world uses of C-dots in forensic science,
specifically their usage in competent fingerprinting for person identification, their importance
in anti-counterfeit campaigns through the development of complicated and difficult-to-
replicate Nanotechnology patterns, and their ability to identify a broad spectrum of biological
substances, illegal narcotics, explosives, toxic chemicals, as well lethal compounds. We hope
to discover both the difficulties and possibilities that are right ahead in this discipline by
critically examining these potential achievements. We aspire to generate efficiencies that will
prepare the way for the growth of environmentally friendly and efficient technologies in the
near future by encouraging interdisciplinary collaboration.
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II. FORENSIC APPLICATIONS
1. Latent Fingerprint Enhancement: Fingerprint examination has been utilized in criminal
investigations for more than a century and is additionally employed to identify
catastrophe victims. It is by far the most extensively used type of biometric
authentication. The method relies on the fact that each person's designs for epidermal
ridges on their fingers are individual and distinctive. Ridge patterns recorded
electronically or with "ink and paper" are saved in national databases, allowing
impressions collected at locations of crime to be compared with the police information
system via a computerized method. Today, the advancement of technologically advanced
scanners has allowed the adoption of fingerprint methods, which are fast gaining traction
in common uses ranging from homeland safety to entry control as well as digital
verification for electronic gadgets.
Initial methods like as ninhydrin, cyanoacrylate fuming, silver nitrate, iodine
vapor, and vacuum metal deposition have endured the test of time and are still utilized in
forensics laboratories. For decades, particles based on titania, carbon, aluminum, silica,
and magnetic particles have been widely used for fingerprint dusting at crime scenes [31].
By introducing powder and spray compositions made from plasmonic nanomaterials,
quantum dots, and C-dots that combine molecular identification agents, excellent quality
imaging, and improved visibility with a strong bond to fingerprint residue, recent
advancements in nanochemistry may lead to a wider range of forensic tools[32]. Cotinine
is the main constituent of nicotine, and nanoparticles of gold carrying anti-cotinine
antibodies, for instance, can produce excellent fingermark imprints while also identifying
it in the fingermarks, giving clear proof of the donor's way of life [33]. A single latent
fingerprints has also been effectively used for multiplexing the identification of illegal
substances and their byproducts using antibody/magnetic nanoparticle conjugates [34].
C-dot-based fingerprint restoration elements shift color when exposed by different
kinds of light, allowing background-free imaging and improving the precision of
fingerprint inspection. C-dots have a propensity to self-quench in their solid form [35].
There have been numerous attempts to mitigate this negative effect, including the use of
diluent matrix structures [36,37], the creation of core-shell small structures [38], the
incorporation of heteroatom doping [39,40], the utilization of Resonance Energy Transfer
(RET) and -interactions [41], the application of molecular gaps [42], and other methods.
The application of C-dot-based particles for the fluorescence visualizing of
invisible fingerprints was first demonstrated by Fernandes et al. [36], who demonstrated
that the addition of 0.7 weight percent C-dots into a silica matrix enabled precise and
color-tunable illustrating of latent fingerprints on a slide made of glass and on a colorful
soft drink label. In the present investigation, the ethanolamine as well as citric acid
monohydrate were heated to create the C-dots, which were then dialyzed against water.
Employing anthracene as a standard, the quantum yield (QY) in water was measured at
15% under 365 nm excitation. XPS examination revealed a mixture of C (44.85%), H
(5.75%), and N (10.85%). This work demonstrates the impressive color-tunability made
possible by exposure with various wavelengths of illumination in addition to the great
level of detail displayed luminously utilizing the C-dot hybrid material. This means that
investigators working on crime scenes can utilize a single powder together with an
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illumination source that can produce varied wavelengths instead of an array of particles to
view fingerprints over a variety of colour backgrounds. Particularly, a fresh fingerprint
created with the hybrid tiny particles disclosed 71 min when compared to 65 min
disclosed from a regular white powder beneath same circumstances.
Li and others [37] Similar encouraging outcomes were obtained for the
fluorescence visibility of latent fingerprints on a range of impermeable surfaces (black
marble, glass, aluminum foil, white ceramic, and a coin) through the incorporation of 1%
C-dots (pyrolytically generated from malic acid and ammonium oxalate) into starch
powder. The C-dot/starch powder outscored the 502 cyanoacrylate glue steam, TiO2
powder, and iodine vapor in a number of situations. This formulation's enhanced
fluorescence was caused by interactions between C-dots and hydroxylgroups that are
present in bulk starch powder.
The focus of additional research by Fernandes and coworkers [38] was
carbogenically-coated silica nanoparticles (C-SiO2) made by combating silica
nanoparticles with dimethyloctadecyl [3- (trimethoxysilyl) propyl] ammonium chloride,
subsequently undergoing pyrolysis, surface oxidation with nitric acid, amine
modification, and finally dialysis contrary to water. The nanoparticles had an average
diameter of 22 nm, 26% C, 4% H, and 5% N. The powder performed well under same
circumstances, adhered well to fingerprints, and showed greater detail (73 minutiae) than
an ordinary white fingerprint powder (65 minutiae). Additionally, although conventional
white powder for fingerprints did not, C-SiO2 nanopowder did, even on highly luminous
cardboard. The commercially available fluorescent powder used did not show any
contrast when lit between 365 and 590 nm, while the C-SiO2 nanopowder did.
According to Wang et al. [39], nitrogen- as well as sulphur-doped C-dots (N, S C-
dots) perform well as color-tunable dusting powders that can reveal enormous amounts of
particulars in latent fingerprints placed on aluminum foil, glass, ceramic, printing paper,
plastic, and steel. They were also similarly successful when used on a fingerprint that had
been on a surface for thirty days. L-glutathione and citric acid were used as precursors via
a microwave-assisted method to create the N, S C-dots, and the finished product was
centrifuged then dialyzed. While TEM showed a size distribution of 2–7 nm, FTIR and
XPS showed the presence of surface functional groups such as OH, NH2, C–O, and SH
that contained oxygen, nitrogen, and sulphur and attributed to their high QY value of
48.1% (in 0.1 M H2SO4 utilizing quinine sulphate as a standard).Milenkovic and others
(40) According to studies, N-doped C-dots (N, C-dots) made hydrothermally by
polyvinylpyrrolidone contain moderately negative charges because OH and COOH are
present in them. As a result, they can magnetically bind to the proteins used in
fingerprints. Excellent quality imprints were found by AFIS examination of a fingerprint
that was generated with N, C-dots and applied to a glossy metal surface (tweezers).
Wang and others, [41] with the aid of a microwave, piperazine and phthalic acid
were pyrolyzed to create graphitic C-dots (pC-dots), which have an average size of 1.5
nm. The pC-dots possess a QY of 20.5% in a solid state and show a very bright yellow-
green color beneath 365 nm light. These extraordinary solid-state luminescence properties
have been correlated to RET and direct contacts. The pC-dots were shown to be efficient
as dusting powders when examined on fingerprints found on weighing paper, a desk
surface, glass, tin foil, plastic, a bottlecap, and a coin.
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Jiang and others [42] It has been stated that polyoxyethylene sorbitan monooleate
distributed in highly concentrated phosphoric acid and concentrated sulfuric acid can be
thermally treated to produce white-emitting C-dots (wC-dots), which are then filtered.
The extremely graphitic nature of the carbogenic cores was confirmed by TEM, XRD,
and Raman spectroscopy, whereas the size of the wC-dots varied from 3.5 to 5.3 nm.
Extended alkyl chains added to the outermost layer of these wC-dots improved
nanoparticle interactions with lipophilic residues present in latent fingerprints while
suppressing aggregation-induced fading. These outcomes might be linked directly to
outstanding dusting fingerprint powder effectiveness of wC-dots.
Deformation, consolidation and separation of substances by a single molecule or
atom are the main processes employed in nanotechnology. The idea of nanotechnology
was put forth by Nobel Prize-winning American physicist Richard Feynman in a 1959
speech titled "There's Plenty of Room at the Bottom" [7]. The term "nanotechnology" was
first coined by researcher Norio Taniguchi in a 1974 paper on the application of synthesis
technology to create objects and properties with nanometer-scale dimensions [8].
The respective inventions of scanning tunneling as well as atomic force
microscopes in the 1980s are viewed as turning events for the development of
nanotechnology as a discipline. These microscopes allowed for the atomic-scale imaging
of materials, a process necessary for changing matter at the atomic and molecular levels.
Supercomputers can now model and analyze compounds on an enormous scale thanks to
concurrent advancements in computer technology, which has led to new discoveries about
the structure and properties of the substances [9]. Research investigations in the 20th
century were significantly impacted by the concurrent modeling, visualizing, and
modifying operations.
2. Anti-Counterfeit: Designer cosmetics, sunglasses, clothing, cigarettes, electrical
appliances, watches, pharmaceuticals, food, oil and electronics are among the items that
are frequently associated with fake goods. The banknotes and other official documents
are also frequently falsified. Since illegal and inferior drugs not only fail to treat patients
but additionally kill thousands of them, counterfeiting and forging are international
crimes that put the wellness and security of customers at risk. In addition to damaging
national economies, this illicit activity and unsettling consumer culture deprives
legitimate firms of valuable resources and tax revenue. Additionally, the production of
unlicensed copies of intellectual property undermines investments in innovation and
scientific research [43,44].
Therefore, effective brand protection & anti-counterfeit laws are essential for
modern societies. Examples of common safety features and graphics include optically
adjustable devices, holograms, laser codes, watermarks, biological and chemical taggants
[45,46]. However, the most intricate designs are easily copied and reproduced. Quantum
dots as well as polyaromatic dyes made from organic materials have been utilized for
security graphics recently [47]; yet, these molecules are extremely expensive, have
adverse reactions, and need solvent-intensive synthesizing methods.
Novel and environmentally friendly nanotechnologies that utilize C-dots have
been developed to help with anti-counterfeiting efforts in order to overcome these
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problems. The fundamental idea behind C-dot-based security nano-barcodes relies on
their distinctive optical attributes, that can result in an unlimited variety of theoretically
unclonable designs and prints that can be virtually impossible to decipher and reverse-
engineer. Fernandes & Company [38]. They proved that the previously characterized C-
SiO2 nanoparticles lacked colloidal stability once the pH level in the media dropped
below 8.5, experienced spontaneous aggregation as their aqueous medium evaporated,
and produced complex structures through a nondeterministic manner. These structures are
difficult to copy, which makes them ideal for labeling for both verification and
identification [48]. The nanotags can theoretically be authenticated against a central
database by scanning them with a mobile device equipped with a magnification and laser
pointer.
A multifunctional anti-counterfeit system that combines photoluminescence,
upconversion photoluminescence (UCPL), and room-temperature phosphorescence (RTP)
was created by Jiang et al. in their study [49] by synthesizing mC-dot dispersions and
incorporating them into a polyvinyl alcohol (PVA) matrix. The m-phenylenediamine was
dissolved in ethanol to create the mC-dots, which were then purified with a silica
chromatography column after being subjected to an extended autoclave boil. Essential
groups like C = N, C-O, and aromatic C-NH2 were detected using analytical methods like
FTIR and XPS. Additionally, XPS examination demonstrated that nitrogen exists in
aminic, pyridinic, and pyrrolic forms. The RTP excitation spectra of the mC-dot/PVA
composite showed a large peak at 360 nm, indicating that the C-N/C = N bond absorption
is the main source of RTP. It is thought that the contact through hydrogen bonding
between the mC-dots and the hydroxyl groups inside the PVA matrix prevents the triplet
state produced by UV light from relaxing vibrationally, leaving room-temperature
phosphorescence (RTP) as the sole possible relaxation mechanism. Three distinctive
characteristics of the anti-counterfeit technology are: First, when subjected to 365 nm
light, the initial marking along with the C-dot marks both fluoresce; second, when a 365
nm light source is turned off, the original marking quickly disappears while the C-dot
markings merely remain visible to the observer due to RTP; and third, when excited with
an 800 nm a laser, cyan-colored markings rather than blue ones are revealed, which is
attributed to upconversion photoluminescence (UCPL).
Carbon dots (C-dots) were created in the work by Kalytchuk et al. [50] by
dissolving citric acid and ethylenediamine in an autoclave made of stainless steel. To
produce "fast" fluorescence lifetime (fC-dots), the procedure was run at 200°C for 5
hours; for "slow" fluorescence lifetime (sC-dots), it was run at 220°C. Investigations
using transmission electron microscopy (TEM) revealed that the average diameter of fC-
dots is 4.7 nm and that of sC-dots is 5.1 nm. Comprehensive FTIR and XPS
investigations revealed the existence of essential chemical linkages, such as C-C, C-OH,
C-N, and N-H bonds. Higher synthesis temperatures resulted in sC-dots containing
greater carbon and lower oxygen contents. Surprisingly, the UV absorption and emission
characteristics of fC-dots and sC-dots were similar, with the main difference being their
estimated fluorescence lifetimes of 7.9 ns for fC-dots and 13.2 ns for sC-dots. Through
encrypting on a paper surface, the use of these C-dots for anti-counterfeiting reasons was
proven. An encrypted version of the letter "R" was printed on the paper, with equal
amounts of either fC-dots or sC-dots ink used to print each pixel. The letter 'R' became
invisible beneath UV light because all pixels emit light at the same rate.
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However, using luminous lifetime images, the symbol 'R' could be easily
decrypted due to those two ink types' unique fluorescence lifetimes. Additionally, by
using different ratios of sC-dots and fC-dots in ink formulations, an untruthful sign, "S,"
could be decoded using UV light, while the real encryption symbol, "K," could only be
seen using fluorescence lifetime imaging.
A microwave-assisted pyrolysis technique using xylose and m-phenylenediamine
(dissolved in water) with the addition of H3PO4 was used by Yang et al. [51] to create
phosphorus-doped carbon dots (P, C-dots), which were then purified using filtration and
dialysis. The end product showed 6.8 nm-diameter graphitic P, C-dots on average. P-O-
(aromatic group) and P-O-H bonds were detected by FTIR analysis, while C-N = C, N-
C3, and N-H bonds were verified by XPS analysis. These P, C-dots had a high quantum
yield (QY = 73.6% in ethanol compared to rhodamine) and emit green light when excited
at 365 nm in the pH range of 3–7. At higher pH values, they change to emit blue/green
light. The group also used these P, C-dots to make a special QR code. The printed QR
code appeared golden yellow in the presence of ambient light, but when exposed to 365
nm radiation, the emission could be dynamically tuned from green to cyan by brushing
the code with a diluted NaOH solution, and the original green emission could be restored
by brushing the code with a diluted CH3COOH solution, offering a reversible and
flexible color adjustment capability.
Hydrophobic C-dots (hC-dots), which change color from blue to red when
exposed to water, were first developed by Yang et al. Under 365 nm illumination, these
evenly distributed hC-dots give off a blue glow, but when water is present, the blue
fluorescence is suppressed by -stacking interactions and transforms into red fluorescence
under 254 nm illumination. The dual-encryption anti-counterfeiting systems are built on
this reversible two-switch photoluminescence (PL) behavior. The letters 'SC,' 'US,' and
'NU' are printed using hC-dot inks in this system, whereas other letters are produced with
standard, non-fluorescent ink. In addition, wax has been applied to the letters "C," "S,"
and "U" to stop water from penetrating them and changing their blue luminescence to red.
A misleading blue luminous code is seen under 365 nm illumination. However, when
water is applied under 254 nm radiation, the real anti-counterfeit markings ('C,' 'S,' and
'U') are visible. Under 254 nm light, no markings are visible in the absence of water.
Single-layer graphene quantum dots (GQDs) with an average size of 2.7 nm were
cleverly contained within stacked double hydroxide (LDH) layers in the research done by
Bai et al. [53]. The precursor, ethylene diamine tetraacetic acid, was co-precipitated into
LDH before being calcined to create the GQD-LDH nanocomposite. It is noteworthy that
the GQDs have surface functions, such as O and N, which promote important interactions
with the LDH host. This GQD-LDH nanocomposite exhibits fluorescence as well as at
room temperature phosphorescence (RTP), giving it exceptional anti-counterfeiting
potential for a variety of applications including food, pharmaceuticals, papers, and
banknotes. In a fascinating presentation, a fluorescent dye was used in conjunction with
the GQD-LDH composite to generate a flower design that was then individually inserted
into gelatin capsules and PVA film.
When these designs were exposed to UV light, they fluoresced, and when the UV
light was turned off, they showed RTP. It's notable that PVA was utilized, a common eco-
friendly packaging material. A previous study by Liu et al. [54] showed the viability of
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adding C-dots to PVA packaging material and recommended it as a practical anti-
counterfeiting strategy.
According to Zhu et al.'s study [35], carbon dot-based fluorescent inks and
composite substances made of C-dots and polymers were employed to create random
fluorescent designs as a successful anti-counterfeiting tactic. Surprisingly, even four
months after preparation, the printed designs and the patterns made with C-dot-polymer
composites both displayed long-term durability. Additionally, even after being exposed to
a strong dosage of 2 kW UV light for 30 minutes, their photoluminescence characteristics
remained unaltered. Furthermore, Sk et al. [55] established the durability of C-dot-based
fluorescent ink on banknotes in a separate research. The ink maintained its visibility even
after being washed with water and then a soap solution, demonstrating its robustness and
potency as a deterrent to counterfeiting.
3. Molecular Sensing: Incorporating integrated microfluidic assays and nanoprobes, lab-on-
a-chip systems are cutting-edge analytical tools that provide multiplexed analysis,
multiplexed detection of several analytes, rapid processing, amazing sensitivity, and low
sample consumption [56]. Because of their small size and user-friendly design, these
portable gadgets don't require specialized knowledge or in-depth training to operate.
Nanosensors have recognition components that are built to target particular molecules and
provide distinctive signals frequently based in optics, electricity, mechanics, or acoustics
[57]. Notably, nanosensors are seen as the future of medical diagnostics, with the ability
to detect underlying illnesses before symptoms appear. They also show great promise in a
variety of other fields, including as forensic toxicology and the detection of explosive
residues.
In order to develop affinity-based nanoprobes, scientists have thoroughly studied a
variety of nanomaterials, including quantum dots [58, 59], plasmonic nanoparticles [60–
63], carbon nanotubes [64], and graphene [65]. These nanoprobes have proven useful in
identifying a variety of chemicals, such as conventional narcotics [58,59], anabolic
steroids [64], pharmaceuticals with misuse potential [62], explosives [60,65], infections
linked to bioterrorism [61], and even in determining the post-mortem interval [63].
Intriguingly, wearable biosensors also provide the capability to continually and in real
time monitor bad lifestyle habits linked to criminal activity, such as binge drinking [66].
Carbon dots (C-dots) are useful in the development of affinity sensors that are
intended to selectively and sensitively bind to particular chemicals in the field of
applications related to forensics. The key sensing methods of these sensors for C-dots are
photo-induced electron transfer (PET), photo-induced charge transfer (PCT), resonance
energy transfer (RET), and inner filter effects (IFE) [67], enabling precise detection.
Detection of illegal chemicals and explosives is a top priority for law enforcement.
Additionally, crucial roles in forensic investigations are played by the detection of
biofluids at crime scenes and the analysis of DNA profiles from these fluids.
Additionally, in the context of forensics, determining if a person has experienced acute or
ongoing exposure to dangerous metals and pesticides could be extremely important,
possibly illuminating homicidal or suicide efforts.
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4. Detection of Biological Compound: Graphitic C-dots (gC-dots) were produced
electrochemically using glycine as the precursor in the study by Wang et al. [68],
resulting in well-controlled particles with a limited size distribution of about 2.4 nm.
Surface functional groups including carboxylate & ammonium were present in these gC-
dots. A constant and linear decrease in the intensity of fluorescence was seen with the
addition of hemoglobin at concentrations ranging from 0.05 to 250 nm, with little
interference from related molecules. Importantly, the gC-dot method was successfully
used to precisely measure the levels of hemoglobin in blood samples, emphasizing the
method's potential for reliable assessment in this situation.
An experiment was done to assess the forensic capability of the gC-dots. Blood
was used toidentify the character on a piece of cloth, which was then washed to make the
character undetectable to the naked sight. When gC-dots were sprayed on the fabric and it
was then subjected to light with a wavelength between 460 and 490 nm, the blood-stained
area showed noticeably less fluorescence than the unaffected parts. Interestingly, no
matter what chemicals were used in this experiment, including proteins, greasy pen ink,
ballpoint pen ink, soy sauce, ketchup, and eggs, the applied gC-dots' fluorescence
behavior remained unchanged. This result emphasizes the haemoglobin sensor's great
selectivity and confirms its potential for forensic applications.
By fusing C-dots via carbon nanotubes (CNTs), Qian et al. [69] created a DNA
nanosensor that can recognize single DNA strands up to 21 base pairs in length. The 2 to
5 nm in diameter graphitic C-dots were created by oxidizing graphite with nitric and
sulfuric acids, then reducing the resultant product with NaBH4 to insert OH and COOH
functional groups, increasing their quantum yield (QY). Strong nitric and sulfuric acids
were used to oxidize the CNTs, which had diameters < 8 nm, and this also resulted in a
decrease in their length. Through a condensation procedure, a terminal amino group at the
5' end of single-stranded DNA (ss-DNA) was added. This modified ss-DNA was then
covalently linked to the surface of the C-dots. The detecting system's functionality works
as follows: Resonance energy transfer (RET) causes fluorescence quenching when CNTs
are introduced because C-dots functionalized with ssDNA attach to the CNT surface. The
C-dot-bound ssDNA, however, base-pairs with the target single-stranded DNA (tDNA),
inducing its release from the CNT surface and restoring the fluorescence of the C-dots.
The target single-stranded DNA is complementary to the strand attached to the C-dots.
The system detection limit is close to 0.4 nM, and the connection between the recovered
fluorescence intensity and tDNA concentration exhibits linearity within a range of 1.5-
133.0 nM.
In contrast to DNA with more CG pairs, a considerable increase in C-dot
fluorescence is seen in the work of Pramanik et al. [70] when double-stranded DNA rich
in AT pairs is present. Notably, the amount of AT pairs and fluorescence intensity have a
linear connection. As a less dangerous option to the staining dyes now in use, the authors
advise adopting this strategy.
Detection Drugs: A fluorescence-quenching approach for identifying
methamphetamine precursors is devised in the study by Kim et al. [71], leveraging the
use of C-dots. These C-dots are created by hydrothermally dealing with a 10:1
mixture of urea and citric acid. The C-dots usually have a size of 2 nm, an amorphous
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core, and both amine and carboxylic acid groups. In particular, the ability to decrease
the fluorescence in the water-based solutions containing these C-dots was shown to be
shared by two methamphetamine precursors, phenylpropane-1,2-diol (PAC-diol) and
phenylpropan-2-one (P2P). It's significant to note that the quenching effect showed a
linear relationship with the concentrations of the individual precursors. Similar results
were obtained with immobilized C-dots on a glass coverslip, which showed that it was
possible to identify these illegal precursors in a liquid dispersion. In contrast to P2P,
that consists of a single carbonyl group, PAC-diol, that has 2 OH groups,
demonstrated a stronger adhesion to the surface of the C-dots, leading to a more
dramatic quenching effect. Importantly, it was discovered that popular diluents such
aspirin, paracetamol, glucose, caffeine, and sodium chloride had no effect on
fluorescence. Amphetamine sulfate, a similar substance, showed a modest decrease in
fluorescence. In addition, chemicals commonly found in illegal drug labs, such as
aniline, benzoic acid, benzyl alcohol, hydroquinone, 4-hydroxybenzoic acid, and 4-
methoxyphenol, had no effect on the fluorescence of the C-dots. According to these
results, the device has a lot of potential as a highly accurate sensing tool for crime
scene investigations.
C-dots, respectively suspended in a water-based solution or coating on paper,
were used in a unique approach developed by Yen et al. [72] for accurate
identification and characterization of cathinones. These L-arginine-derived graphitic
C-dots had an average diameter of 4.4 nm, and a hydrothermal technique was used to
provide surface functional groups with oxygen and nitrogen during their production.
Both heroin and cocaine showed a little drop in C-dot emission at pH 7, which is
probably because of their low solubility. Still, only the cathinones showed appreciable
quenching of fluorescence at pH 11. This distinction between heroin and cocaine,
which have ester groups that are conjugated, and cathinones, which have non-
conjugated ketones, suggests that these are the main substances causing the quenching
effect that has been observed. Particularly, non-conjugated ketones had little to no
impact on the fluorescence of the C-dots. Under pH 11 conditions, the system showed
a detection limit of 1.73 mm (0.43 mg/mL-1) for 4-chloroethcathinone, demonstrating
the possibility for sensitive detection.
A portable UV light with a wavelength of 254 nm, C-dot-impregnated paper,
and a mobile phone camera were used in a more usable detecting setup for the
chemical. This setup displayed an impressive detection limit of 0.14 mm, or 0.03
mg/mL-1. The coated paper's fluorescence showed a linear decline proportional to the
4-chloroethcathinone concentration in the range of 0.5-10.0 mM. Importantly, high
glucose concentrations had no effect on the signal transmitted, indicating that this
detection device had exceptional selectivity for this particular molecule. The use of C-
dot-impregnated paper also included the ability to find 4-chloroethcathinone in urine.
With a limit of detection of 1300 ng mL-1 in this application, the paper-based
approach showed a linear response over the concentration range of 2000-12,500 ng
mL-1. The feasibility, selectivity, and sensitivity of the C-dot-based paper technique
in identifying this particular chemical, notably in urine samples, are highlighted by
these results.
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Detection of Explosives: With the aid of C-dots, several research teams have created
sensitive and focused fluorescence-quenching detectors for explosive aromatic nitro
compounds like picric acid (PA). Niu et al.'s research involved the creation of
amorphous C-dots with diameters ranging from 4 to 6 nm. The manufacture of these
C-dots, which have surface amine & carbonyl groups, involved microwave processing
with equal parts (3 g each) of citric acid and urea. A significant photoluminescence
(PL) quenching effect was produced by the addition of PA to a C-dot dispersion,
allowing for a detection limit of roughly 1 M. Notably, the addition of numerous
common interferents had no effect and neither did the structurally similar molecules
2,4-dinitrotoluene (DNT) nor 2,4,6-trinitrotoluene (TNT). Similar detection
techniques involved exposing sealed containers containing C-dot-impregnated paper
to the fumes of DNT, TNT, and PA for 600 seconds. Immediately after the container
was opened, the accompanying PL spectra were gathered. The PL quenching in this
case was only 49.8% for PA due to PA's lower vapor pressure compared to 77% for
DNT and 80.3% for TNT. These findings offer useful information for applications
involving explosive detection by showcasing the capability of C-dots to selectively
identify PA in varied configurations.
By pyrolyzing ammonium citrate dibasic using a standard microwave, Sun et
al. [74] showed a technique that produced C-dots with a size distribution between 5.5
and 1.5 nm and an extremely disordered structure. Citric acid, N-doped C-dots, and
low-molecular-weight oligomers made up the final mixture. This mixture was used
immediately after centrifugation to eliminate big clumps without any additional
purification. In contrast, negligible impact is seen with the comparable nitro aromatic
explosives, the estimated detection limit for this configuration was 0.25 M, and the
addition of increasing amounts of PA systematically caused the C-dot fluorescence to
be quenched.
The greater sensitivity of the C-dots for PA above the other nitro compounds
can be explained by PA's lower LUMO than that of the other nitro compounds and the
electron-deficient nature of nitro compounds, which leads to PL quenching from PET.
Comparable values of PA selectivity and sensitivity were observed by Siddique et al.
[75], They employed C-dots made by ultrasonically processing a mixture of dextrose
and HCl with a variety of O-containing surface functional groups but no nitrogen.
In order to create C-dots using activated carbon, Campos et al. [76] used a
multistep oxidation procedure, which was followed by functionalization with a poly
(amidoamine) (PAMAM-NH2) dendrimer. Only the explosive nitro compound 4-
chloro-2,6-dinitroaniline (4-Cl-2,6-DNA) caused the formation of a second emission
band at 507 nm among the chemicals studied. Furthermore, unlike several comparable
compounds, this one specifically muted the C-dots' 465 nm fluorescence noticeably.
The ratio of fluorescence at these two wavelengths showed a linear association with
the concentration of 4-Cl-2,6-DNA over the concentration range of 1.0 10-5 to 6.0 10-
4 M. The potential for selective detection and quantification of the explosive molecule
is highlighted by this creative method, opening the door for useful applications in this
field.
Pal et al. [77] introduced a composite film made of C-dots and polypyrrole
(PPy) for conductivity-based PA sensing. To start the polymerization of pyrrole, this
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novel method used C-dots, which were made from ethylene diamine and citric acid.
Larger C-dots were found to be separately coated with PPy, as confirmed by TEM
imaging of the final product, whereas smaller C-dots, average a diameter of 4.5–2 nm,
got integrated into the PPy matrix. Selected area electron diffraction (SAED) was
used to confirm the material's semi-crystalline structure, and current-versus-voltage
(I-V) graphs showed that the film's metal-like conductivity was the result of
interconnected networks of C = C bonds. The composite film showed a maximum
conductivity of 2.60 mS m-1 in contrast to PPy's conductivity of 0.23 mS m-1.
Notably, in contrast to its negligible effects on related compounds, aqueous PA
considerably increased the conductivity of this nanocomposite. According to research,
the detection threshold for PA is 1.40 10-7 M. Additionally, the detection limit for PA
incorporation was discovered to be 5.7 ng mg-1 when soil samples were analyzed.
These results highlight the capability of this composite film to detect PA even in
intricate sample matrices in a delicate and practical manner.
Detection of Heavy Metals and Pesticides: In order to produce PEGylated N-doped
C-dots, Gupta et al. [78] microwave-heated a chitosan gel while adding PEG and
dithiothreitol (DTT) functionalization. AFM and TEM measurements of the
unfunctionalized C-dots revealed an average diameter of about 8 nm. They had a wide
variety of O- and N-containing functional groups on their surfaces, with N making up
only 2.46% of the total mass. DTT's binding and the existence of free SH groups
inside of it were both confirmed by XPS and FTIR analyses. These C-dots showed
water solubility, retained stability even at high salt concentrations, and fluoresced at
their greatest levels at physiological pH. Hg2+ ions were added, and this considerably
decreased the fluorescence intensity while having no discernible impact on the
emission intensity. The DTT-functionalized C-dots, on the other hand, displayed
exceptional sensitivity, detecting concentrations as low as 18 pm in distilled water, 45
pm in filtered (0.22 m), centrifuged, spiked river water, and 50 pM in spiked tap
water. However, for Hg2+ in distilled water, unfunctionalized PEGylated C-dots
obtained a detection limit of 6.8 nm. The strong interaction between the thiol groups
and Hg2+ is responsible for the functionalized C-dots' high sensitivity. These findings
highlight the functionalized C-dots' improved performance, making them an
advantageous tool for delicate detection applications.
Citric acid and cysteamine were used as precursors in the investigation
conducted by D. et al. [79] to synthesize C-dots, which were then functionalized using
DTT. SAED and XRD studies of the C-dots during characterisation confirmed their
low crystallinity index. The C-dots' size was determined by high-resolution TEM to
be between 4-5 nm. Different surface functional groups with O, N, and S were visible
on these C-dots. The presence of free SH groups following functionalization and the
binding of DTT via S-S linkages were further confirmed by FTIR analysis. With a
remarkable low limit of detection close to 0.086 ppb, it is noteworthy that the
presence of As3+ ions was discovered to increase the fluorescence intensity of the C-
dots in a Tris-HCl buffer. This sensing method's effectiveness was shown by its
ability to find As3+ in spiked water samples taken from wells, lakes, and the ground.
While the As-O bond, which is normally stronger, tends to dissolve in water, the
DTT-functionalized C-dots' extraordinary selectivity for As3+ over other metal ions
was attributable to its high strength (379 kJ/mol). This method has a great deal of
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potential for the sensitive and focused detection of As3+, offering insightful data for
environmental monitoring and research.
A ratiometric, dual-emission nanosensor for Cu2+ ions were created by Liu et
al. [80] employing rhodamine-B-doped silica nanoparticles covered with C-dots. N-
(aminoethyl)-aminopropylmethyldimethoxysilane (AEAPMS) was pyrolyzed to
create the C-dots, and citric acid was then added. The remaining ethylenediamine
groups on the C-dot surface were able to successfully capture Cu2+ ions without
further alteration, while the leftover methoxysilane groups from AEAPMS made it
easier to attach to dye-doped silica nanoparticles. According to XPS examination, the
silica nanoparticles had a diameter of roughly 145 nm before the C-dots were
attached, while the C-dots as-prepared had a diameter of 2-3 nm in ethanol. Being
illuminated at 360 nm, the resultant hybrid material showed unique dual PL emissions
at 467 and 585 nm, coming from the contributions of C-dots and rhodamine B dye,
respectively. When Cu2+ ions in the concentration range of 0–10.0 M were added, the
C-dot fluorescence contribution was significantly reduced, but the dye signal barely
changed. Up to 3 10-6 M, a linear decrease in the fluorescence intensity ratio at 467
and 585 nm was seen as the Cu2+ concentration rose within this range. This device
demonstrated its sensitivity with a computed detection limit of 35.2 nm using a signal-
to-noise ratio of 3. When Cu2+ was present, changes in pH between pH 5.0 and 10.0
had little effect on the fluorescence ratio. The method had exceptional selectivity and
worked well for measuring Cu2+ in biological samples and tainted water. C-dots have
previously been shown to be sensitive and selective for the detection of lead ions in
metal ion detection applications when mounted on spherical polyelectrolyte brushes.
For a variety of analytical and environmental monitoring applications, this novel
technique shows promise [81], While a fluorescent probe for mercury detection
constructed of a polyaniline/C-dot nanocomposite has been developed [82].
Li et al.'s [83] development of an organophosphorus pesticide (OP) detection
system using red-emissive C-dots made through hydrothermal processing of a
combination of thiourea and citric acid. NaOH and HCl were used in order to improve
the C-dots' optical characteristics. While TEM pictures showed diameters in the range
of 4–7 nm, indicating the presence of six to twelve layers of sheets resembling
graphene, the height of the C-dots, as assessed by AFM, varied from 1.8 to 3.8 nm.
The C-dots' graphitic origin was validated by FTIR analysis, XRD, and Raman
spectroscopy, as well as several surface functional groups involving O, N, and S. The
C-dots displayed strong PL emission at 610 nm under 550 nm excitation and were
soluble in a number of organic solvents, including ethanol, DMSO, methanol, and
DMF. The foundation of the OP ratiometric sensor was created when dopamine (DA)
was polymerized into polydopamine (PDA) in the presence of C-dots. This signal at
503 nm quenched the PL of the C-dots via PET. With the addition of OPs (paraoxon,
parathion, and malathion), the contribution from PDA was increased while the
contribution from C-dots was decreased. Paraoxon, parathion, and malathion were
determined to have permitted residue limits of 0.125 pg mL-1, 0.0625 pg mL-1, and
0.025 pg mL-1, respectively. It's significant that the sensor system demonstrated
remarkable selectivity because it was unaffected by a variety of ions, amino acids,
proteins, and other pesticide families. Analysis of spiked samples of tap water, river
water, soil, rice, apple, and serum was used to verify the validity of the sensing
system. The adaptability and promise of C-dots in pesticide detection applications are
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shown by other C-dot-based approaches with sensitivity focusing on detecting methyl
parathion [84] and Dichlorvos or DDVP, also known as 2,2-dichlorovinyl dimethyl
phosphate [85].
III. CHALLENGES AND FUTURE PERSPECTIVES
1. Latent Fingerprint Enhancement: According to a survey of the literature, materials
scientists have been principally in charge of this field's research, with a noticeable focus
on the photo-optical performance of powders rather than their usefulness for fingerprint
identification. It is clear that close cooperation between materials scientists and forensic
investigators/practitioners is essential to the development of this broad and extremely
challenging subject. The International Fingerprint Research Group's experimental
methods and overarching ideas could be widely adopted to make substantial progress in
this regard [86]. Notably, when looking at the stages of development, it seems that most
articles that have been published belong to phase 1 pilot studies, with only a small
number appearing in phases 2 (optimization and comparison), 3 (validation), and 4
(operational evaluation and casework trials) [87].
Furthermore, it is crucial to ensure the protection of forensic operations, which
necessitates thorough evaluations of the potential health concerns and long- and short-
term toxicity linked to C-dot-based devices. Although this area is yet understudied, it
should be formed to solve concerns with rigorous preparation processes, controlled
drying, and the intrinsic ability of C-dots to cling strongly to diluent materials. Before the
legal community can safely accept this new category of nanomaterials, such assurance is
necessary.
It is necessary to conduct a thorough analysis to determine whether newly created
C-dot-based formulations, which include both powders and dispersions, are effective at
recovering fingerprints from a variety of substrates, including porous and non-porous
surfaces. The cases when fingerprints are exposed to harsh environmental conditions or
are polluted with bodily fluids should be included in this examination. Further research
should be done to determine whether these formulations have the ability to find DNA
within lifted fingerprints. It is likely that C-dot formulations used for fingerprint
generation may also be able to identify metabolites and provide insights into numerous
behavioral and lifestyle aspects, similar to the abilities seen with gold nanoparticles.
2. Anti-Counterfeit: A growing body of research indicates that ecologically benign C-dot-
based nanotags can be made with little upfront cost, offering a workable alternative for
underdeveloped countries struggling with the rising manufacture of fake goods. C-dot-
based formulations have a significant amount of promise for use in anti-counterfeiting
applications, and when compared to competing technologies, their comparatively easy
and affordable synthesis presents an advantage. However, novel approaches for mass
production of high-security inks and nanotags are still in the research and development
stage, and a number of fundamental issues need to be fully resolved before a workable
manufacturing model can be developed and implemented.
C-dot-based authentication patterns must be thoroughly examined for structural
integrity, response to various spectra, resistance to humidity, and adaptation to poor
storage circumstances, as well as for compatibility with a variety of common supporting
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substrates. Additionally, it is important to explore improvements in printing methods that
are adapted to the particular demands of each targeted business, including the use of ink
formulations that are already in common use. Digital engineers will need to make major
contributions in order for technology to progress for the collection, processing, and
analysis of encrypted images, opening the door for the creation of a robust, automated,
and trustworthy anti-counterfeit system.
3. Molecular Sensing: The absence of established techniques in the field of nanomaterials
continues to be a major obstacle, which is especially obvious in the application of
forensic nanosensors. Unfortunately, this undermines the validity and reproducibility of
published experimental findings, making meaningful comparisons with one another and
with accepted industry norms difficult to accomplish [88]. The implementation of basic
reporting standards for nanosensors is actively supported by a number of projects [89].
This calls for careful and thorough synthetic processes as well as a thorough structural
analysis of the nanoparticles that takes into account factors like elemental composition,
shape, porosity size, surface area, hydration level, surface functionalities, dispersity index
zeta potential and the presence of impurities. It is also necessary to give accurate
analytical performance measures, such as the linearity range, detection limit,
repeatability, stability, and reproducibility.
Strong experimental data supports the benefits of C-dot-based sensors, but a more
methodical approach is required to realize these benefits in practical applications. This
requires controlling the elemental composition, size, and crystallinity of C-dots as well as
fine-tuning the density of surface functional groups on C-dots to regulate bandgap and
quantum yield. Comprehensive performance evaluations of C-dot nanosensors are crucial,
especially in complex biofluids like urine, blood, stomach contents, aqueous humor and
spinal fluid. To ensure resistance against signal interference and decrease false positives
and false negatives. Realizing their practical utility also requires specifying the best
storage conditions for them and demonstrating how they may be integrated into
microfluidic arrays, lab-on-a-chip and automated detection systems.
IV. CONCLUSIONS
C-dots are a rapidly developing class of nano-emitters, albeit it is still not quite clear
what they are. These cutting-edge materials have the power to transform criminal justice and
law enforcement practices, assisting contemporary civilizations in the fight against crime,
illegal activity and terrorism. Due to their photoluminescent characteristics, C-dots can be
used as sensitive nano-probes in forensic toxicology. This enables extremely responsive
detection of analytes such illegal substances, pesticides, heavy metals and explosives.
Intelligent fingerprinting can greatly benefit from the adaptive color-tuning behavior of C-
dot-enriched powders, which gets around problems with increased background interference.
Additionally, C-dot-based dyes and polymeric compositions are advantageous as nano-tags
for object identification as well as authentication because they have the rare capacity to
produce security prints that are very impossible to predict or clone. A collaborative effort
including researchers and practitioners from several domains is necessary to fully use the
promising body of experimental evidence, which is predominantly driven by proof-of-
concept demonstrations.
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