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Exfoliation of Egyptian Blue and Han Blue, Two Alkali Earth Copper Silicate-based Pigments

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Darrah Johnson-McDaniel
Darrah Johnson-McDaniel
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Tina T. Salguero at University of Georgia
Tina T. Salguero
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In a visualized example of the ancient past connecting with modern times, we describe the preparation and exfoliation of CaCuSi4O10 and BaCuSi4O10, the colored components of the historic Egyptian blue and Han blue pigments. The bulk forms of these materials are synthesized by both melt flux and solid-state routes, which provide some control over the crystallite size of the product. The melt flux process is time intensive, but it produces relatively large crystals at lower reaction temperatures. In comparison, the solid-state method is quicker yet requires higher reaction temperatures and yields smaller crystallites. Upon stirring in hot water, CaCuSi4O10 spontaneously exfoliates into monolayer nanosheets, which are characterized by TEM and PXRD. BaCuSi4O10 on the other hand requires ultrasonication in organic solvents to achieve exfoliation. Near infrared imaging illustrates that both the bulk and nanosheet forms of CaCuSi4O10 and BaCuSi4O10 are strong near infrared emitters. Aqueous CaCuSi4O10 and BaCuSi4O10 nanosheet dispersions are useful because they provide a new way to handle, characterize, and process these materials in colloidal form.
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Video Article
Exfoliation of Egyptian Blue and Han Blue, Two Alkali Earth Copper Silicate-
based Pigments
Darrah Johnson-McDaniel1, Tina T. Salguero1
1Department of Chemistry, The University of Georgia
Correspondence to: Tina T. Salguero at salguero@uga.edu
URL: http://www.jove.com/video/51686
DOI: doi:10.3791/51686
Keywords: Chemistry, Issue 86, Nanosheets, Egyptian Blue, Han Blue, Pigment, Near Infrared, Luminescence, Exfoliation, Delamination, Two-
Dimensional, Ink, Colloidal Dispersion
Date Published: 4/24/2014
Citation: Johnson-McDaniel, D., Salguero, T.T. Exfoliation of Egyptian Blue and Han Blue, Two Alkali Earth Copper Silicate-based Pigments. J. Vis.
Exp. (86), e51686, doi:10.3791/51686 (2014).
Abstract
In a visualized example of the ancient past connecting with modern times, we describe the preparation and exfoliation of CaCuSi4O10 and
BaCuSi4O10, the colored components of the historic Egyptian blue and Han blue pigments. The bulk forms of these materials are synthesized
by both melt flux and solid-state routes, which provide some control over the crystallite size of the product. The melt flux process is time
intensive, but it produces relatively large crystals at lower reaction temperatures. In comparison, the solid-state method is quicker yet requires
higher reaction temperatures and yields smaller crystallites. Upon stirring in hot water, CaCuSi4O10 spontaneously exfoliates into monolayer
nanosheets, which are characterized by TEM and PXRD. BaCuSi4O10 on the other hand requires ultrasonication in organic solvents to achieve
exfoliation. Near infrared imaging illustrates that both the bulk and nanosheet forms of CaCuSi4O10 and BaCuSi4O10 are strong near infrared
emitters. Aqueous CaCuSi4O10 and BaCuSi4O10 nanosheet dispersions are useful because they provide a new way to handle, characterize, and
process these materials in colloidal form.
Video Link
The video component of this article can be found at http://www.jove.com/video/51686/
Introduction
Vibrant colors were prized throughout the ancient world. Even today, we can still see the remains of pigments and dyes created by every major
culture. Remarkably, two of the most famous synthetic blue pigments share a similar chemical composition and structure, despite having been
developed at widely different times and places. The colored components of both Egyptian blue, CaCuSi4O10, and Han blue, BaCuSi4O10, belong
to the alkali earth copper tetrasilicate series, ACuSi4O10 (A = Ca, Sr, Ba)1, as well as the larger gillespite group, ABSi4O10 (B = Fe, Cu, Cr)2,3.
Beyond traditional pigment applications, current scientific interest in these materials centers on their strong near infrared (NIR) emission
properties. This emission originates from the Cu2+ in square planar coordination; these ions are linked by tetrahedral silicate moieties within the
three-dimensional crystal structure, and the resulting layers alternate with alkali earth ions4-6. Recent technical highlights include NIR imaging to
identify Egyptian and Han blue pigments on cultural heritage artifacts7,8, lanthanide doping of ACuSi4O10 to enhance NIR reflectance properties
and open new energy transfer pathways9,10, the use of ACuSi4O10 as the active material for optical sensors11, and the exfoliation of CaCuSi4O10
into monolayer nanosheets12.
In particular, this last example provides a way to nanostructure CaCuSi4O10 so that it can be handled as a colloidal dispersion rather than as a
particulate solid12. Because colloidal dispersions are compatible with solution-processing techniques (e.g. spin coating, ink jet printing, layer-
by-layer deposition), this advance opens new application areas that range from security inks to biomedical imaging. The experimental protocols
illustrated in this contribution will enable researchers from diverse backgrounds to prepare, characterize, and use CaCuSi4O10 and BaCuSi4O10
nanosheets in their work.
Protocol
1. Preparation of CaCuSi4O10
1. Melt Flux Synthesis of CaCuSi4O10
1. Weigh out CaCO3, SiO2, and Cu2CO3(OH)2 in a 2:8:1 molar ratio: 0.1331 g (1.330 mmol) of CaCO3, 0.3196 g (5.319 mmol) of SiO2,
0.1470 g (0.6648 mmol) of Cu2CO3(OH)2. In addition, weigh out the flux components (12.5% by weight): 0.0375 g of Na2CO3, 0.0125 g
of NaCl, and 0.0250 g of Na2B4O7.10H2O. Add these materials to a clean agate mortar.
2. Hand grind for ~5 min with an agate pestle until the mixture becomes a homogeneous light green powder (Figures 1a and 2a).
Transfer this mixture to a clean, dry platinum crucible.
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3. Heat the crucible in a furnace to 875 °C (ramp rate of 2 °C/min), hold at 875 °C for 16 hr, and then cool down to room temperature (rate
of 0.8 °C/min).
4. Remove the crystals from the crucible and gently crush them using a pestle.
5. Allow the crystals to soak in 50 ml of 1 M aqueous HCl overnight to remove the melt flux.
6. Filter the crystals and wash with deionized water to fully remove any remaining melt flux.
Note: This material should be ground into a finer powder for powder X-ray diffraction (PXRD) analysis (Figure 5). It also can be
characterized by optical microscopy (Figure 3), scanning electron microscopy (SEM) (Figure 4), and NIR photography (Figure 8).
2. Solid State Synthesis of CaCuSi4O10
1. Weigh out CaCO3, SiO2, and CuO in a 1:4:1 molar ratio: 0.1331 g (1.330 mmol) of CaCO3, 0.3196 g (5.319 mmol) of SiO2, and 0.1058
g CuO (1.330 mmol) and add to a clean agate mortar.
2. Dampen the powder mixture with 1-2 ml acetone and hand grind with an agate pestle for ~5 min. Transfer the resulting light gray
powder (Figures 1b and 2b) into a platinum crucible.
3. Heat the crucible in a box furnace to 1,020 °C at a ramp rate of 5 °C/min, hold for 16 hr, and then cool down to room temperature
4. Scrape out the loose, light blue-gray powder using a polytetrafluoroethylene (PTFE) spatula.
Note: The product can be characterized by optical microscopy (Figure 3), SEM (Figure 4), PXRD (Figure 5), and NIR photography
(Figure 8).
2. Synthesis of BaCuSi4O10
1. Melt Flux Synthesis of BaCuSi4O10
1. Weigh out BaCO3, SiO2, and CuO in a 1:4:1 molar ratio: 0.2085 g BaCO3 (1.057 mmol), 0.2539 g SiO2 (4.226 mmol), and 0.0840 g
CuO (1.056 mmol). In addition, weigh out the flux component (12.5% by weight): 0.0765 g of PbO. Add these materials to a clean
agate mortar.
2. Hand grind for ~5 min with an agate pestle until the mixture becomes a homogeneous light gray powder (Figures 1c and 2c). Transfer
this mixture to a clean, dry platinum crucible.
3. Heat the crucible in a furnace to 950 °C (ramp rate of 2 °C/min), hold at 950 °C for 24 hr, then slowly cool down to 700 °C (rate of 0.1
°C/min), and finally cool to room temperature.
4. Remove the crystals from the crucible and gently crush them using a pestle.
5. Allow the crystals to soak in 50 ml of 1 M aqueous HNO3 overnight to remove the melt flux.
6. Filter the crystals and wash with deionized water to fully remove the remainder of the melt flux. Note: This material should be ground
into a finer powder for PXRD analysis (Figure 6). It also can be characterized by optical microscopy (Figure 3) and NIR photography
(Figure 8).
2. Solid State Synthesis of BaCuSi4O10
1. Weigh out BaCO3, SiO2, and CuO in a 1:4:1 molar ratio: 0.2085 g BaCO3 (1.057 mmol), 0.2539 g SiO2 (4.226 mmol), and 0.0840 g
CuO (1.056 mmol) and add to a clean agate mortar.
2. Dampen the powder mixture with 1-2 ml acetone and hand grind with an agate pestle for ~5 min. Transfer the resulting light gray
powder (Figures 1d and 2d) into a platinum crucible.
3. Heat the crucible in a box furnace to 960 °C at a ramp rate of 5 °C/min and hold for 16 hr, then cool to room temperature.
4. Scrape out the loose blue powder using a polytetrafluoroethylene (PTFE) spatula. Note: The product can be characterized by optical
microscopy (Figure 3), PXRD (Figure 6), and NIR photography (Figure 8).
3. Exfoliation of CaCuSi4O10
1. Charge a 50 ml round bottom flask with 0.50 g of CaCuSi4O10, 40 ml of deionized water, and a glass-coated magnetic stir bar.
2. Attach a water-cooled condenser to the flask. Heat the reaction to 85 °C with magnetic stirring at 400 rpm for two weeks.
3. Remove from the heat source, allow the solution to settle undisturbed overnight, and then filter the supernatant through a 0.4 µm membrane
filter. Vacuum dry the solids. Note: The product is a light blue powder that can be characterized by optical microscopy (Figure 3), PXRD
(Figure 5), transmission electron microscopy (TEM) (Figure 7), and NIR photography (Figure 8).
4. Exfoliation of BaCuSi4O10
1. Charge a 50 ml plastic centrifuge tube with 0.14 g of BaCuSi4O10 and 20 ml of N-vinyl pyrrolidone.
2. With the centrifuge tube immersed in an ice/water bath, sonicate with a probe ultrasonicator at 40% amplitude (17 W) for 1 hr.
3. Let the dispersion settle undisturbed overnight, and then decant the supernatant into a new centrifuge tube.
4. Spin down at 10,286 x g using a centrifuge. Decant the supernatant, leaving the nanosheets at the bottom of the centrifuge tube.
5. Resuspend this material in 20 ml of water with a few minutes of bath sonication. To isolate a powder, filter through a 0.4 µm membrane
filter and vacuum dry the solids. Note: The product is a light blue powder that can be characterized by optical microscopy (Figure 3), PXRD
(Figure 6), TEM (Figure 7), and NIR photography (Figure 8).
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5. Ink Preparation
1. Disperse ~0.10 g of CaCuSi4O10 nanosheets in 5 ml of deionized water using bath sonication for ~10 min. Note: This ink (Figure 9) can be
used for painting, printing, etc. See Figure 10 for a representative example where the ink was applied to paper with a brush.
6. Near Infrared Photographic Imaging
1. Irradiate the samples using red light (e.g. with a red light-emitting diode array), taking care to eliminate any other sources of light.
2. Photograph using a camera modified to image in the near infrared region. Use f stop setting f/22 and an exposure time of 0.5 sec.
Representative Results
The described syntheses of CaCuSi4O10 and BaCuSi4O10 provide approximately 0.5 g of product per batch. Isolated yields of CaCuSi4O10 from
the melt flux and solid-state syntheses typically range from 70-75% and 90-95%, respectively. For BaCuSi4O10, the isolated yields from the melt
flux and solid-state syntheses typically range from 65-70% and 95-99%, respectively.
The textures of all of the prepared materials, as well as differences in the intensity of their blue color due to varying crystallite sizes, are visible
by low magnification optical microscopy (Figures 3a-h). Scanning electron microscopy (SEM) images confirm that the solid-state method
of synthesizing CaCuSi4O10 produces ~1-15 µm primary crystallites (Figure 4b) whereas melt flux conditions lead to ~5-50 µm crystallites
(Figure 4a). Powder X-ray diffraction (PXRD) patterns for CaCuSi4O10 (Figures 5a and 5c) and BaCuSi4O10 (Figures 6a and 6c) showcase the
composition and phase purity of these products.
Representative transmission electron microscopy (TEM) images show the nanosheet morphology of the exfoliated products (Figure 7). In
addition, NIR photographic imaging shows the strong luminescence of both the bulk and exfoliated materials (Figure 8). A simple way to
illustrate the solution processability of CaCuSi4O10 nanosheets is to prepare an aqueous ink (Figure 9) suitable for painting (Figure 10).
Figure 1. Photographs of the hand-ground starting materials. (a) CaCuSi4O10 melt flux, (b) CaCuSi4O10 solid-state, (c) BaCuSi4O10 melt flux,
and (d) BaCuSi4O10 solid-state syntheses. Please click here to view a larger version of this figure.
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Figure 2. Scanning Electron Microscopy. Images of the hand-ground starting materials for the (a) CaCuSi4O10 melt flux, (b) CaCuSi4O10 solid-
state, (c) BaCuSi4O10 melt flux, and (d) BaCuSi4O10 solid-state syntheses. All samples were coated with gold prior to imaging. Please click here
to view a larger version of this figure.
Figure 3. Optical Microscopy. Bulk CaCuSi4O10 prepared by melt flux (a) and solid state (b) procedures. Bulk BaCuSi4O10 prepared by melt
flux (c) and solid state (d) procedures. Exfoliated products (e-h) of (a-d), respectively. All images share the 1 mm scale bar show in panel (a).
Please click here to view a larger version of this figure.
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Figure 4. Scanning Electron Microscopy. Images of bulk CaCuSi4O10 made by melt flux (a) and solid state (b) methods. Samples were coated
with gold prior to imaging. Please click here to view a larger version of this figure.
Figure 5. Powder X-Ray Diffraction: CaCuSi4O10. Patterns for bulk CaCuSi4O10 prepared by melt flux (a) and solid state (c) methods. Asterisks
denote a silica impurity. Patterns for exfoliated CaCuSi4O10, (b) and (d), prepared from (a) and (c), respectively. Please click here to view a larger
version of this figure.
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Figure 6. Powder X-Ray Diffraction: BaCuSi4O10. Patterns for bulk BaCuSi4O10 prepared by melt flux (a) and solid state (c) methods. Asterisk
denotes a silica impurity. Patterns for exfoliated BaCuSi4O10, (b) and (d), prepared from (a) and (c), respectively. Please click here to view a
larger version of this figure.
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Figure 7. Transmission Electron Microscopy. Representative images of exfoliated CaCuSi4O10 derived from bulk CaCuSi4O10 made by melt
flux (a) or solid state (b) methods. Representative images of exfoliated BaCuSi4O10 derived from bulk BaCuSi4O10 made by melt flux (c) or solid
state (d) methods. Please click here to view a larger version of this figure.
Figure 8. Near Infrared Imaging. Luminescence of bulk CaCuSi4O10 prepared by melt flux (a) and solid state (b) procedures. Luminescence of
bulk BaCuSi4O10 prepared by melt flux (c) and solid state (d) procedures. Luminescence of the exfoliated products (e-h) of (a-d), respectively.
Powder samples are contained within glass vials, and the entire set of samples was imaged at once. Please click here to view a larger version of
this figure.
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Figure 9. Photograph of a CaCuSi4O10 nanosheet ink in a vial.
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Figure 10. Near Infrared Imaging. A rudimentary painting with the CaCuSi4O10 nanosheet ink that illustrates both its simple application and its
luminescence properties.
Discussion
The preparation of Egyptian blue pigment, a mixture of mostly CaCuSi4O10 and SiO2, is a well-studied process4,13-21. The numerous reported
procedures may be categorized as either melt flux or solid-state reactions. Two major advantages of the melt flux approach are that it permits
lower reaction temperatures (<900 °C) and allows CaCuSi4O10 crystals to nucleate and grow from a molten glass phase20. The flux component
is typically an alkali salt (e.g. Na2CO3) or borate compound (e.g. borax). In comparison, the solid-state syntheses omit the flux but require higher
temperatures (~1,000 °C) for the reaction between Ca, CuO, and SiO2 sources to reach completion.
Although the synthesis of Han blue pigment is not as well studied as that of Egyptian blue4,22-25, the preparation of BaCuSi4O10 follows similar
melt flux and solid-state routes with two differences: (1) a PbO flux should be used, and (2) the reaction temperatures must be more closely
controlled because of alternative Ba-Cu-Si-O phases that can form (e.g. BaCuSi2O6).
These points are illustrated by the detailed procedures and results described in this paper. First, for all methods, the starting materials should
be ground to a smooth powder (Figures 1a-d) consisting of 5-20 µm particles (characterized by SEM; Figures 2a-d). Next, the use of a
significant amount of flux (12.5% by weight) in the preparation of CaCuSi4O10 and BaCuSi4O10 leads to highly crystalline products, which are
characterized by intense blue coloration (Figures 3a and 3c), relatively large particle sizes (Figure 4a), and strong PXRD patterns (Figures
5a and 6a). The diminished isolated yields (~70%) from these preparations are caused by adhesion of the melted reaction mixtures to the
crucible. In comparison, CaCuSi4O10 and BaCuSi4O10 prepared by the solid-state route exhibit less intense coloration (Figures 3b and 3d) and
smaller particle sizes (Figure 4b). As synthesized, these products are powders that can be isolated in near-quantitative yields. Thus, for both
CaCuSi4O10 and BaCuSi4O10, the advantages of flux and the importance of reaction temperature cannot be overstated.
Remarkably, the exfoliation of CaCuSi4O10 and BaCuSi4O10 occurs under simple aqueous conditions. In the case of CaCuSi4O10, this reaction is
quite slow at room temperature (≥6 weeks to see any appreciable exfoliation), but it becomes synthetically useful at 80 °C (substantial exfoliation
after 2 weeks). In comparison, the exfoliation of BaCuSi4O10 is sluggish even at 80 °C, and so we apply an even greater energy input in the
form of ultrasonication. These reactions are highly reliable with two caveats. For CaCuSi4O10, it is important to use a glass-coated stir bar; if a
standard PTFE-coated stir bar is used, we find that PTFE byproducts contaminate the CaCuSi4O10 nanosheet product. For BaCuSi4O10, it is
important to control the ultrasonication power and time so that the reaction is stopped before the nanosheets become degraded.
Transmission electron microscopy (TEM) of the nanosheet products shows that these very thin materials have lateral dimensions ranging from
hundreds of nanometers to several microns. In general these lateral dimensions correlate with the crystallite size of the three-dimensional
starting material. In prior work, atomic force microscopy provided topographic mapping that demonstrated the single-layer thicknesses (~1.2 nm)
of these nanosheets12. Photographs of powder CaCuSi4O10 and BaCuSi4O10 nanosheet samples (Figures 3e-h) show that their color is less
intense than that of the starting materials, a direct result of nanostructuring.
Additional information is provided by PXRD (Figures 5 and 6), which reveals basal cleavage along the (001) plane and preferred orientation
along the {00l} series for all nanosheet samples. These features reflect the stacked alignment of these highly anisotropic nanomaterials when
drop-cast onto a substrate. Furthermore, the characteristic NIR emission of CaCuSi4O10 at ~910 nm and BaCuSi4O10 at ~950 nm is illustrated in
a NIR photograph of all eight samples (Figure 8).
The solution processing of CaCuSi4O10 can be accomplished by simply preparing a colloidal dispersion of CaCuSi4O10 nanosheets (Figure 9)
to use as an ink. This ink then can be applied to a substrate via spin coating, spray coating, ink jet printing12, or simply brushing (Figure 10).
Importantly, the NIR emission properties of CaCuSi4O10 are retained at all stages of this process. These new possibilities highlight the contrast
between CaCuSi4O10 nanosheets and the traditional use of Egyptian blue pigment, a highly granular material that is challenging to incorporate
into a smooth paint.
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Disclosures
The authors have no competing financial interests.
Acknowledgements
We thank Prof. Mark Abbe (UGA) for providing the NIR imaging equipment and Dr. Rasik Raythatha (Solvay Performance Chemicals) for the
barium carbonate used in this work. We acknowledge the efforts of Isaiah Norris (UGA undergraduate) and Terra Blevins (North Oconee High
School), who helped test the synthetic methods.
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Asian Science, Technology, and Medicine. 17, 94-120 (2000).
... The chemical formula of HP is predominantly BaCuSi₂O₆, and it stands out not only for its relatively straightforward synthesis but also for its fluorescent properties, attributed to the presence of Cu²⁺ ions within its structure [2] . The synthesis process involves the combination of precursors such as barium carbonate (BaCO₃), silicon dioxide (SiO₂), and copper oxide (CuO) under precise thermal control [3,4] . These unique characteristics allow HP to emit fluorescence within the near-infrared spectrum (NIR), unlocking new possibilities for its application in biomedical sensors, fluorescent markers, and infrared imaging techniques [2,5] . ...
... Synthesis of HP The HP pigment was produced by combining barium carbonate (BaCO₃), silicon dioxide (SiO₂), and copper oxide (CuO) in a 1:2:1 molar ratio, following the procedure described by McDaniel et al. (2014) [4] . The components were homogenized using a highspeed mixer (SpeedMixer™ -Hauschild DAC 250), at 1100 rpm for 5 minutes. ...
... Following this step, the powder was deagglomerated and exposed to a second thermal cycle at 1050°C with a heating rate of 5°C/min. After high-temperature processing, the material gradually cooled to room temperature (25°C) [4] . The overall equation (Eq. 1) describing the chemical reaction involves the decomposition of BaCO₃ and the subsequent formation of the fluorescent BaCuSi₂O₆ phase of HP: BaCO 3(s) + 2SiO 2(s) + CuO (s) → BaCuSi 2 O 6(s) + CO 2(g) (1) X-ray Diffraction (XRD) X-ray diffraction measurements were performed using a Rigaku Miniflex 600. ...
Photoluminescent gels based on han purple: new frontiers in biotechnology
Article
Full-text available
  • Dec 2024
As a result of the increasing need for new biocompatible materials, polymer-based gels have become promising options. Lately, photoluminescent gels have shown potential for applications in biotechnology and non-invasive tracking due to their ability to emit light when temperature changes occur. This research investigates the incorporation of Han Purple (HP) pigment into a polyethylene glycol/Laponite matrix [92,5/7,5%] (P7LHP0%) to produce a 3D-printed gel. The gels were examined for possible use as smart sensors, focusing on their optical and thermal characteristics. Formulations with HP concentrations of 0.0%, 0.5%, and 2.0% were prepared, followed by extrusion-based 3D printing. Characterization techniques included FTIR, SEM, and optical analyses (emission and excitation spectra). The findings showed 3D structures with good shape fidelity while FTIR indicated suitable compatibility between HP and the matrix. Optical analysis revealed fluorescence with an excitation band between 400 and 700 nm, with a maximum at 620 nm, and an emission band at 830 - 1000 nm with a peak at 925 nm. This study highlights the potential of HP as a promising material for fluorescent gels in 3D printing, creating new opportunities for biotechnology applications
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... "cool" pigments) [2,[18][19][20], phosphors [21], forensic science [22], and luminescent solar concentrators [23]. In 2013, the development of a method to exfoliate EB and produce 2D photoluminescent nano-sheets [24] further increased the interest in the pigment, expanding the family of 2D materials [25][26][27]. This newly discovered feature opened the way to applications in next-generation fields such as security inks [28,29] and especially biological imaging and biomedicine [5,[30][31][32][33]. ...
... Melt-flux methods (also reported as salt-flux syntheses) imply the use of a flux (e.g. soda or plant ash) that lowers the melting point of silica, allowing the production of cuprorivaite in the 800-900 • C temperature range [15,24]. ...
... Cuprorivaite can also be produced through a direct solid-state synthesis without the use of any flux, by raising the temperature to about 1000 • C [24,39,40]. In this case the main processes involved can be considered surface diffusion of Ca and Cu ions on the surface of silica particles [38,41]. ...
Increased NIR photoluminescence of Egyptian blue via matrix effect optimization
Article
Full-text available
  • Feb 2024
  • MATER CHEM PHYS
HIGHLIGHTS • We report solid-state synthesis of cuprorivaite starting from silica nanoparticles. • The synthetized material displays an exceptionally high quantum yield (Φ EM ≈ 30 %). • The synthesized tiny crystals are almost devoid of any copper-rich glassy phase. • We demonstrate how limiting the glassy phase increases the external quantum efficiency. • The results can greatly boost the use of cuprorivaite in energy saving and biomedicine. ARTICLE INFO Keywords: Egyptian blue - Cuprorivaite - NIR photoluminescence - Quantum yield - Silica nanoparticles - Solid-state synthesis - Melt-flux synthesis ABSTRACT The exceptionally high NIR photoluminescence of the ancient pigment Egyptian blue is due to its very stable phase cuprorivaite (CaCuSi4O10). This compound has recently attracted significant attention, leading to numerous applications for sensors, luminescent solar concentrators, energy-saving, and biomedicine. Here we report an innovative manufacturing process for producing high-grade cuprorivaite, characterized by fine crystal grains and a significantly increased NIR photoluminescence emission. The unprecedented ultra-high NIR emission (quantum yield Φ EM ≈ 30 %) is almost three times higher than the best one reported so far. This is an important turning point for the extension of applications of cuprorivaite to new sectors and can greatly boost its exploitation. The new high-efficiency cuprorivaite is obtained by solid-state synthesis, using silica nanoparticles as a starting material and avoiding fluxing agents. No doping with rare-earths or other elements has been employed, making synthesis straightforward and sustainable. The material obtained has been fully characterized in terms of crystalline, morphological, and optical properties and compared to cuprorivaite obtained through traditional melt-flux synthesis. The main difference observed is that the tiny crystals obtained through the new synthesis method are practically devoid of the glassy phase, rich in copper and impurities, that is instead largely present in Egyptian Blue pigment synthesized with traditional melt-flux synthesis. We speculate that this glassy phase is responsible for the partial suppression of the intrinsic photoluminescence of cuprorivaite, demonstrating how limiting the glassy phase can increase the external quantum efficiency of Egyptian blue.
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... Table 1 presents a summary of the main attempts reported in the literature. Concerning the amount of the raw materials, the stoichiometric proportions are by now attested, whereas the flux is not always added to the batch, proceeding in some experiments with a solid-state synthesis [19,20,22]. The temperatures tested range from 760 to 1200 • C for highly variable duration (from 2 to 24 h) and heating cycles. ...
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... These pigments are alkaline earth copper silicates, which are often manufactured using solid-state chemical methods. In a nutshell, the oxides or carbonates of the component metals are well mixed and then heated in air to a temperature approaching 900 • C for a few hours [140]. It is common for the dark compound copper oxide to remain as an impurity after the synthesis. ...
Sustainable Mitigation Strategies for Urban Heat Island Effects in Urban Areas
Article
Full-text available
  • Jul 2023
The globe is at a crossroads in terms of the urban heat island effect, with rising surface temperatures due to urbanization and an expanding built environment. This cause-and-effect connection may be linked to weather-related dangers, natural disasters, and disease outbreaks. Urbanization and industrialization will not lead to a secure and sustainable future. Finding solutions to problems such as the heat island effect is at the forefront of scientific research and policy development. Sustainable ways to decrease urban heat island impacts are a core principle for urban planners. This literature study examines the benefits of adding green infrastructure and sustainable materials in built-up areas to reduce the urban heat island effect. Materials such as reflective street pavements, coating materials including light-colored paint, phase-change materials, color-changing paint, fluorescence paint, and energy-efficient appliances are considered sustainable materials, whereas green infrastructure like green roofs, green walls, green parking and pavements, and shaded streets are considered to mitigate the urban heat island effect. The hurdles to the widespread adoption of such practices include a lack of governmental legislation, insufficient technological development, an erroneous estimation of economic gains, and unwillingness on the part of impacted parties.
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... From the viewpoint of material synthesis conditions, the glazed pottery contains elements like Ba, Cu, Si, and Pb in the glaze layer. It is available to synthesize copper barium silicate under the condition of raw materials and synthesis temperature [28,29]. There are a series of simulation synthesis studies carried out by several scholars in China and abroad [30,31]. ...
Scientific and technical analysis of lead-barium glaze dragonfly eye beads from the Late Warring States period in China
Article
Full-text available
  • Sep 2023
This paper analyzed five dragonfly eye beads excavated from M176 of the Hejia Cemetery in the Late Warring States period (around 3rd c. BC) by using a super depth of field 3D microscope system (OM), scanning electron microscope-energy dispersive spectrum (SEM–EDS) and Raman spectroscopy. The analytical results confirmed that all the beads were glazed pottery and the glaze material belongs to the lead-barium-silicate (PbO-BaO-SiO2) system. The color component of the glaze is Chinese Blue (BaCuSi4O10). Three beads, M176-2, M176-3, and M176-4, were formed with an inner core support and were made in the same batch. Additionally, two weathering products, CuPb4(SO4)2(OH)6 and PbCO3, were detected on the glaze layer surface. The results of scientific and technological analysis show that these beads have differences in the composition of the body and glaze, and the color composition in the glaze layer is relatively rare in previous studies. The discovery of lead-barium glazed pottery beads from the Late Warring States period in northern China provides new evidence for further exploration into the origins and evolution of early glazed pottery. The identification of weathering products formed on the beads’ surface within an alkaline burial environment holds valuable implications for the study of weathering and deterioration in silicate artifacts.
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Real-time identification and visualization of Egyptian blue using modified night vision goggles
Article
Full-text available
  • Apr 2024
The possibility to use light in the visible spectrum to induce near-infrared luminescence in some materials, particularly Egyptian blue and related pigments, offers a significant advantage in terms of their detection. Since 2008, this property has been exploited to reveal the presence of those pigments even in tiny amounts on ancient and decayed surfaces, using a technical-photography method. This paper presents a new type of imaging device that enables real-time, easy, and inexpensive identification and mapping of Egyptian blue and related materials. The potential of the new tool is demonstrated by its effectiveness in detecting Egyptian blue within some prestigious sites: (a) Egyptian findings at Museo Egizio, Turin; (b) underground Roman frescoes at Domus Aurea , Rome; and (c) Renaissance frescoes by Raphael, Triumph of Galatea and Loggia of Cupid and Psyche, at Villa Farnesina, Rome. The device is based on night vision technology and allows an unprecedented fast, versatile, and user-friendly approach. It is employable by professionals including archeologists, conservators, and conservation scientists, as well as by untrained individuals such as students or tourists at museums and sites. The overall aim is not to replace existing photographic techniques but to develop a tool that enables rapid preliminary recognition, useful for planning the work to be carried out with conventional methods. The ability to immediately track Egyptian blue and related pigments, through real-time vision, photos, and videos, also provides a new kind of immersive experience (Blue Vision) and can foster the modern use of these materials in innovative applications and future technologies.
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Real-time identification and visualization of Egyptian blue using modified night vision goggles
Preprint
Full-text available
  • Mar 2024
The possibility to use light in the visible spectrum to induce near-infrared luminescence in some materials, particularly Egyptian blue and related pigments, offers a significant advantage in terms of their detection. Since 2008 this property has been exploited to reveal their presence even in tiny amounts on ancient and decayed surfaces, using a technical-photography method. This paper presents a new type of imaging device that enables real-time, easy, and inexpensive identification and mapping of Egyptian blue and related materials. The potential of the new tool is demonstrated by showing its effectiveness in detecting Egyptian blue within some prestigious sites: a) Egyptian findings at Museo Egizio, Turin, b) underground Roman frescoes at Domus Aurea , Rome, and c) Renaissance frescoes by Raphael, Triumph of Galatea and Loggia of Cupid and Psyche , at Villa Farnesina, Rome. The device is based on night vision technology and allows an unprecedented fast, versatile, and user-friendly approach. It is employable by professionals including archaeologists, conservators, and conservation scientists, as well as by un-trained individuals such as students or tourists at museums and sites. The overall aim is not to replace existing photographic techniques but to develop a tool that enables rapid preliminary recognition, useful for planning the work to be carried out with conventional methods. The ability to immediately track Egyptian blue and related pigments, through real-time vision, photos, and videos, provide also a new kind of immersive experience (Blue Vision) and can foster the modern use of these materials in innovative applications and future technologies.
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Surfactant assisted exfoliation of near infrared fluorescent silicate nanosheets
Article
Full-text available
  • Jul 2023
Fluorophores that emit light in the near infrared (NIR) are advantageous in photonics and imaging due to minimal light scattering, absorption, phototoxicity and autofluorescence in this spectral region. The layered silicate Egyptian blue (CaCuSi4O10) emits as a bulk material bright and stable fluorescence in the NIR and is a promising NIR fluorescent material for (bio)photonics. Here, we demonstrate a surfactant-based (mild) exfoliation procedure to produce nanosheets (EB-NS) of high monodispersity, heights down to 1 nm and diameters <20 nm in large quantities. The approach combines planetary ball milling, surfactant assisted bath sonication and centrifugation steps. It avoids the impurities that are typical for the harsh conditions of tip-sonication. Several solvents and surfactants were tested and we found the highest yield for sodium dodecyl benzyl sulfate (SDBS) and water. The NIR fluorescence emission (λem ≈ 930-940 nm) is not affected by this procedure, is extremely stable and is not affected by quenchers. This enables the use of EB-NS for macroscopic patterning/barcoding of materials in the NIR. In summary, we present a simple and mild route to NIR fluorescent nanosheets that promise high potential as NIR fluorophores for optical applications.
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Synthesis, Properties and Mineralogy of Important Inorganic Materials
Book
Full-text available
  • Jan 2021
ntended as a textbook for courses involving preparative solid-state chemistry, this book offers clear and detailed descriptions on how to prepare a selection of inorganic materials that exhibit important optical, magnetic and electrical properties, on a laboratory scale. The text covers a wide range of preparative methods and can be read as separate, independent chapters or as a unified coherent body of work. Discussions of various chemical systems reveal how the properties of a material can often be influenced by modifications to the preparative procedure, and vice versa. References to mineralogy are made throughout the book since knowledge of naturally occurring inorganic substances is helpful in devising many of the syntheses and in characterizing the product materials. A set of questions at the end of each chapter helps to connect theory with practice, and an accompanying solutions manual is available to instructors. This book is also of appeal to postgraduate students, post-doctoral researchers and those working in industry requiring knowledge of solid-state synthesis.
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Forward and Back Energy Transfer Between Cu2+ and Yb3+ in Ca1−xCuSi4O10:Ybx Crystals
Article
Full-text available
  • Nov 2012
We report on near-infrared photoluminescence studies in Ca1−xCuSi4O10: Ybx (x = 0.00 − 0.10) polycrystals by means of diffuse reflection, photoluminescence, excitation spectra and luminescence decay analysis. The samples show intense absorption bands at 540, 630, 800 nm due to Cu2+: 2B1g→2A1g, 2Eg, 2B2g transitions. Under the excitation, the samples show efficient photoluminescence at 920 nm from Cu2+ and 1007 nm from Yb3+, respectively. With increasing of Yb concentration, the PL intensity of Yb3+ increases obviously, while that of the Cu2+ decreases slightly. A probable energy transfer mechanism can be proposed on the basis of decay curves and temperature-dependent photoluminescence spectra.
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The synthetic Cr (super 2+) silicates BaCrSi 4 O 10 ; the missing links in the gillespite-type ABSi 4 O 10 series
Article
  • Aug 1997
The new Cr (super 2+) -containing silicate compounds BaCrSi 4 O 10 and SrCrSi 4 O 10 were synthesized both from alkali-borate fluxes and by high-T subsolidus solid-state reactions. The gillespite-type crystal structures (space group P4/ncc, Z = 4) were determined from single-crystal X-ray diffraction data. The unit-cell parameters are a = 7.4562(4), c = 15.5414(4) Aa for SrCrSi 4 O 10 , and a = 7.5314(3), c = 16.0518(4) Aa for BaCrSi 4 O 10 . Comparison with previously published data shows that A (= Ba, Sr, Ca) cation substitution in ABSi 4 O 10 gillespite-type compounds mainly affects the c lattice parameter whereas the substitution of the B (= Cu, Cr, Fe) site leads to only small changes, mainly in a. The Cr (super 2+) cation occupies a square-planar coordinated site unique in oxide crystal chemistry, with a Cr-O bond length of 1.999+ or -0.002 Aa in all three Cr compounds. The rigidity of these bonds leaves the CrSi 4 O 10 layers within the structure with only one significant degree of freedom, that of rotation of the four-membered Si 4 O 10 rings in response to substitution on the A cation site. The magnitudes of these rotations are independent of the identity of the B cation. In addition the AO 8 polyhedron becomes more elongated //c with increasing radius of the A cation. The increasing aplanarity of the O(3)X 3 , configuration is almost exclusively determined by occupational changes on A, whereas the aplanarity of the square-planar BO(3) 4 group can be related to the positional shifts induced by the individual substitutions on both A and B sites. Polarized optical absorption spectroscopy was conducted on (hk0) sections of SrCrSi 4 O 10 and BaCrSi 4 O 10 .
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Structure of BaCuSi4O10
Article
  • Jul 1992
Cristallisation dans P4/ncc avec a = 7,440 et c = 16,097 A, Z = 4; affinement jusqu'a R = 0,0202. La structure consiste en cycles de quatre tetraedres SiO 4 lies. Chaque cycle est connecte a quatre autres pour former une couche silicate dans le plan ab. Ce compose est un isotype synthetique du mineral gillespite.
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The synthetic Cr2+ silicates BaCrSi4O10 and SrCrSi4O10: The missing links in the gillespite-type ABSi4O10 series
Article
  • Jul 1997
The new Cr2+-containing silicate compounds BaCrSi4O10 and SrCrSi4O10 were synthesized both from alkali-borate fluxes and by high-T subsolidus solid-state reactions. The gillespite-type crystal structures (space group P4/ncc, Z = 4) were determined from single-crystal X-ray diffraction data. The unit-cell parameters are a = 7.4562(4), c = 15.5414(4) Å for SrCrSi4O10, and a = 7.5314(3), c = 16.0518(4) Å for BaCrSi4O10. Comparison with previously published data shows that A (= Ba, Sr, Ca) cation substitution in ABSi4O10 gillespite-type compounds mainly affects the c lattice parameter whereas the substitution of the B (= Cu, Cr, Fe) site leads to only small changes, mainly in a. The Cr2+ cation occupies a square-planar coordinated site unique in oxide crystal chemistry, with a Cr-O bond length of 1.999 ± 0.002 Å in all three Cr compounds. The rigidity of these bonds leaves the CrSi4O10 layers within the structure with only one significant degree of freedom, that of rotation of the four-membered Si4O10 rings in response to substitution on the A cation site. The magnitudes of chese rotations are independent of the identity of the B cation. In addition the AO, polyhedron becomes more elongated // c with increasing radius of the A cation. The increasing aplanarity of the O(3)X, configuration is almost exclusively determined by occupational changes on A, whereas the aplanarity of the square-planar BO(3)4 group can be related to the positional shifts induced by the individual substitutions on both A and B sites. Polarized optical absorption spectroscopy was conducted on (hk0) sections of SrCrSi4O10 and BaCrSi4O10. Absorption bands at ∼19500, ∼14900, and ∼22070 cm-1 could be assigned to 5B1g → 5B2g, 5B1g → 5A1g (E ⊥ c), and 5B1g → 5Eg, (E // c) spin-allowed d-d transitions for Cr2+ in a square-planar configuration. The crystal-field stabilization energies of 13110 ± 150 and 13220 ± 180 cm-1 are indistinguishable for both compounds reflecting the very similar CrO4 geometries.
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Lead Zirconate Titanate PbZr0.52Ti0.48O3 by a Coprecipitation Method Followed by Calcination
Chapter
  • Jan 2011
Intended as a textbook for courses involving preparative solid-state chemistry, this book offers clear and detailed descriptions on how to prepare a selection of inorganic materials that exhibit important optical, magnetic and electrical properties, on a laboratory scale. The text covers a wide range of preparative methods and can be read as separate, independent chapters or as a unified coherent body of work. Discussions of various chemical systems reveal how the properties of a material can often be influenced by modifications to the preparative procedure, and vice versa. References to mineralogy are made throughout the book since knowledge of naturally occurring inorganic substances is helpful in devising many of the syntheses and in characterizing the product materials. A set of questions at the end of each chapter helps to connect theory with practice, and an accompanying solutions manual is available to instructors. This book is also of appeal to postgraduate students, post-doctoral researchers and those working in industry requiring knowledge of solid-state synthesis.
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Technological Examination of Egyptian Blue
Chapter
  • Jan 1984
The chemical composition, microstructure, hardness, and color of a series of ancient Egyptian Blue samples from Egypt, Mesopotamia, and Western Europe (Roman period) have been investigated using principally atomic absorption spectrophotometry and scanning electron microscopy. Egyptian Blue has been produced in the laboratory by using a range of compositions and firing procedures and has been compared with the ancient material. From these results, information on the techniques used in antiquity to produce Egyptian Blue has been obtained. In particular, it seems probable that a two-stage firing cycle with grinding and molding to the final shape between the first and second firing was used in the production of small objects.
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Lanthanum–strontium copper silicates as intense blue inorganic pigments with high near-infrared reflectance
Article
  • Sep 2013
Novel non-toxic intense blue near-infrared reflecting inorganic pigments having the general formula Sr1–xLaxCu1–yLiySi4O10 (x = y ranges from 0 to 0.5) were developed as viable alternatives to existing toxic cobalt based blue colorants. The pigment powders were characterized by XRD and UV–vis–NIR diffuse reflectance spectroscopy. The substitution of La3+ for Sr2+ and Li+ for Cu2+ in SrCuSi4O10 gently changes the color of the pigment from sky-blue to intense blue and as a result the band gap of the pigment powders increases from 2.59 to 2.68 eV. The coloring mechanism is based on the crystal field splitting of the Cu2+ d-orbitals in a square planar environment. Finally the ability of the pigments to transfer the color as well as NIR reflectance properties was demonstrated by coating on to concrete cement block and PMMA. Most importantly, the developed pigments exhibit intense blue color with impressive NIR solar reflectance (67%) and thermally stable.
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