The present invention relates in general to starch-stabilized gold nanoparticle aqueous dispersion with a narrow range of hydrolyzed starch and narrow size distribution of the gold nanoparticles. More specifically, the present invention relates to a novel process for synthesis and formulation of starch-stabilized gold nanoparticles and their dispersion in water, wherein the concentrated dispersion produces a dried film by ink-jet printing or related printing techniques, and that is highly conducting and that sinters at low temperature to increase its conductivity. According to the present invention, a specific range of molecular weights of hydrolyzed starch yields a stable starch-gold nanoparticle dispersion with a low starch-to-gold mass ratio and a monodisperse size distribution of the gold nanoparticles.
Printed electronics is one of the recent advances in manufacturing various electronic devices where in inkjet printing, the material is only deposited on specific areas on the substrate thus saving on cost of materials [Kang, J. S.; Kim, H. S.; Ryu, J.; Thomas Hahn, H.; Jang, S.; Joung, J. W. J., Mater. Sci. Mater. Electron. 2010, 21 (11), 1213-1220]. Inkjet printing has been employed to print conductive patterns made from metallic electrodes such as silver and gold [Cui, W.; Lu, W.; Zhang, Y.; Lin, G.; Wei, T.; Jiang, L., Colloids Surfaces A Physicochem. Eng. Asp. 2010, 358 (1-3), 35-41; Jensen, G. C.; Krause, C. E.; Sotzing, G.; Rusling, J. F., Phys. Chem. Chem. Phys. 2011, 13 (11), 4888-4894; Volkman, S. K.; Yin, S.; Subramanian, V. Mater. Res. Soc. Proc. 2004, 814, 17.8]. A major component in inkjet printing technology is the ink formulation that will be jetted through the nozzle into single isolated droplets. Jettable inks can be formulated by varying its fluid properties, specifically the surface tension, density and viscosity. A common criterion for jettability of inks is determined by the dimensionless parameter called Z-number, which is the reciprocal of the Ohnesorge number [Ohnesorge, W. V. J. Appl. Math. Mech. 1936, 16 (6), 355-358]. This parameter relates the viscous forces of the fluid to its inertial and surface tension forces, as shown in the equation below,
Z=(σρd)1/2/η (Eq. 1)
where σ is surface tension, ρ is density, and η is viscosity of the ink, while d is the nozzle diameter. However, several Z-number ranges (e. g., 1<Z<10 or 4≤Z≤14)18 have been reported by different authors for the jettability window. Hence, some studies have actually defined the jettability regime for inks using various two-parameter plots [Kim, E.; Baek, J. Model. Phys. Fluids 2012, 24 (8)]. For example, in the study by Subramanian et al., a jettable ink formulation is empirically defined within regions bounded by the capillary and Weber (Ca-We) number space, highlighting the effects of inertial and viscous forces normalized by surface tension of the ink formulation; these regions appear to be universal for jettable ink formulations that produce satellite-free droplets [Nallan, H. C.; Sadie, J. A.; Kitsomboonloha, R.; Volkman, S. K.; Subramanian, V. Langmuir 2014, 30, 13470-13477].
For printing the metallic electrodes like silver and gold, the inks are either colloidal suspension of their nanoparticles or salt precursor solutions which can be post-processed to achieve the desired conductive metal [Määttänen, A.; Ihalainen, P.; Pulkkinen, P.; Wang, S.; Tenhu, H. J.; Peltonen, ACS Appl. Mater. Interfaces 2012, 4 (2) 955-964; Huang, D.; Liao, F.; Molesa, S.; Redinger, D.; Subramanian, V. J. Electrochem. Soc. 2003, 150 (7), G412; Kosmala, A.; Zhang, Q.; Wright, R.; Kirby, P., Mater. Chem. Phys. 2012, 132 (2-3), 788-795; Shen, W.; Zhang, X.; Huang, Q.; Xu, Q.; Song, W., Nanoscale 2014, 6, 1622-1628; Perelaer, J.; de Laat, A. W. M.; Hendriks, C. E.; Schubert, U. S., J. Mater. Chem. 2008, 18 (27), 3209; Jahn, S. F.; Blaudeck, T.; Baumann, R. R.; Jakob, A.; Ecorchard, P.; Rüffer, T.; Lang, H.; Schmidt, P., Chem. Mater. 2010, 22; Magdassi, S.; Kamyshny, A; Vinetsky, Y; Bassa, A.; Mokh, R. A., US 2005/0078158A1]. In ink formulations of metal nanoparticle suspensions, aside from jettability, size and stability are also important factors to consider. The nanoparticles are usually stabilized with a capping agent which gives a very stable dispersion on the selected solvent, which is usually nonpolar. In the case of gold nanoparticles (AuNP), it is commonly synthesized using the Brust process which is a two-phase synthesis with NaBH4 as the reducing agent and alkanethiols, or surfactants, as the capping agent [Cortie, M. B.; Coutts, M. J.; Ton-That, C.; Dowd, A.; Keast, V. J.; McDonagh, A. M., J. Phys. Chem. C 2013, 117 (21), 11377-11384]. Since the capping agents usually have nonpolar groups, it is suspended in a mixture of organic solvents. Greener approach to AuNP synthesis has also been a trend wherein more environmentally-benign reducing agents and stabilizing ligands are used in the synthesis. There are already many studies which explored green synthesis of silver and gold nanoparticles using plant extracts [Elia, P.; Zach, R.; Hazan, S.; Kolusheva, S.; Porat, E.; Zeiri, Y. Int. J. Nanomedicine 2014, 9, 4007-4021; Ovais, M.; Raza, A.; Naz, S.; Islam, N. U.; Khalil, A. T.; Ali, S.; Khan, M. A.; Shinwari, Z. K. Appl. Microbiol. Biotechnol. 2017, 101 (9), 3551-3565; Dauthal, P.; Mukhopadhyay, M., 3 Biotech 2016, 6 (2), 1-9; Godipurge, S. S.; Yallappa, S.; Biradar, N. J.; Biradar, J. S.; Dhananjaya, B. L.; Hegde, G.; Jagadish, K.; Hegde, G. A., Enzyme Microb. Technol. 2016, 95, 174-184], natural polymers [Ban, D. K.; Pratihar, S. K.; Paul, S. RSC Adv. 2015, 5 (99), 81554-81564; Pooja, D.; Panyaram, S.; Kulhari, H.; Reddy, B.; Rachamalla, S. S.; Sistla, R. Int. J. Biol. Macromol. 2015, 80, 48-56.], and other alternatives to nonpolar ligands and solvents [Cui, W.; Lu, W.; Zhang, Y.; Lin, G.; Wei, T.; Jiang, L. Colloids Surfaces A Physicochem. Eng. Asp. 2010, 358 (1-3), 35-41; Zeng, S.; Du, L.; Huang, M.; Feng, J. X. Bioprocess Biosyst. Eng. 2016, 39 (5), 785-792; Liu, J.; Qin, G.; Raveendran, P.; Ikushima, Y. Chem.—A Eur. J. 2006, 12 (8), 2131-2138].
Of special interest is the use of starch as both the reducing and stabilizing agent for gold nanoparticles (AuNPs) [Vantasin, S.; Pienpinijtham, P.; Wongravee, K.; Thammacharoen, C.; Ekgasit, S. Sensors Actuators, B Chem. 2013, 177, 131-137; Tajammul Hussain, S.; Iqbal, M.; Mazhar, M. J. Nanoparticle Res. 2009, 11 (6), 1383-1391; Engelbrekt, C.; Sorensen, K. H.; Zhang, J.; Welinder, A. C.; Jensen, P. S.; Ulstrup, J., J. Mater. Chem. 2009, 19, 7839; Raveendran, P.; Fu, J.; Wallen, S. L. Green Chem. 2006, 8 (1), 34]. Microwave-assisted synthesis and hydrolysis had been reported in the synthesis of these nanoparticles [Pienpinijtham, P.; Thammacharoen, C.; Ekgasit, S. Macromol. Res. 2012, 20 (12), 1281-1288; Arshi, N.; Ahmed, F.; Kumar, S.; Anwar, M. S.; Lu, J.; Koo, B. H.; Lee, C. G. Curr. Appl. Phys. 2011, 11, S360-S363; Rastogi, L.; Arunachalam, J. Int. J. Green Nanotechnol. 2012, 4 (2), 163-173; Kundu, S.; Peng, L.; Liang, H. Inorg. Chem. 2008, 47 (14), 6344-6352].
However, none of these documents known in the art provide the teaching according to the present invention, wherein the starch solution is hydrolyzed by microwave-assisted heating in NaOH solution to activate its reducing ends. This will in turn reduce the Au ions in the solution to Au(0) wherein an optimized molecular weight range of hydrolyzed starch molecules stabilize the nanoparticles to control its further growth. Size-controlled and stable AuNP dispersions are then produced from this technique without the use of nonpolar ligands and solvents.
The present invention relates in general to starch-stabilized gold nanoparticle aqueous dispersion with a narrow range of hydrolyzed starch and narrow size distribution of the gold nanoparticles. More specifically, the present invention relates to a novel process for synthesis and formulation of starch-stabilized gold nanoparticles and their dispersion in water, wherein the concentrated dispersion produces a dried film by ink-jet printing or related printing techniques, and that is highly conducting and that sinters at low temperature to increase its conductivity. According to the present invention, a specific range of molecular weights of hydrolyzed starch yields a stable starch-gold nanoparticle dispersion with a low starch-to-gold mass ratio and a monodisperse size distribution of the gold nanoparticles.
The primary object of this invention is to provide an aqueous AuNP ink formulation that uses an optimally hydrolyzed starch, obtained through microwave-assisted heating that controls the size and reducing capacity of said optimally hydrolyzed starch.
It is also the object of this invention to provide an aqueous AuNP ink formulation that uses an optimally hydrolyzed starch, obtained through microwave-assisted heating, as both the reducing and stabilizing agent in the microwave synthesis of the AuNP nanoparticles.
Still another object of this invention is to provide an aqueous AuNP ink formulation that can be used in inkjet printing for printed electronics applications, as alternative to commercial AuNP ink formulations that are organic-solvent based.
Yet another object of this invention is to provide a process for optimized synthesis of AuNP based on the particle size, stability, and yield by varying the starch hydrolysis conditions carried out by microwave-assisted hydrolysis and reaction, in which the microwave-assisted heating provides more uniform heating to the reaction mixture.
Furthermore, it is another object of the present invention to provide a process for an optimized AuNP synthesis reaction using microwave-assisted heating to yield a narrow, monodisperse size distribution of AuNP resulting to printed Au films having low sheet resistance (<1.0 Ω/square) even at low sintering temperatures (ca. 200° C.).
These and other objects will become apparent upon reading the following detailed description taken in conjunction with the accompanying drawings.
The present invention relates in general to starch-stabilized gold nanoparticle aqueous dispersion with a narrow range of hydrolyzed starch and narrow size distribution of the gold nanoparticles. More specifically, the present invention relates to a novel process for synthesis and formulation of starch-stabilized gold nanoparticles and their dispersion in water, wherein the concentrated dispersion produces a dried film by ink-jet printing or related printing techniques, and that is highly conducting and that sinters at low temperature to increase its conductivity. According to the present invention, a specific range of molecular weights of hydrolyzed starch yields a stable starch-gold nanoparticle dispersion with a low starch-to-gold mass ratio and a monodisperse size distribution of the gold nanoparticles.
The new approach to aqueous AuNP ink formulation according to the present invention uses an optimally hydrolyzed starch by microwave-assisted heating (to control size and reducing capacity) and using such as both the reducing and stabilizing agent in the subsequent microwave synthesis of the AuNP nanoparticles. The ink formulation according to the present invention can be used in inkjet printing, in relation to application in printed electronics, as alternative to most commercial AuNP ink formulations that are organic-solvent based (
The approach by Ekgasit et al. [Pienpinijtham, P.; Thammacharoen, C.; Ekgasit, S. Macromol. Res. 2012, 20 (12), 1281-1288; Arshi, N.] uses hydrolyzed starch for the reduction of Au3+ to Au0, wherein they proposed that the hydrolysis of starch under mild alkaline conditions produces intermediates containing either an aldehyde or a-hydroxy ketone, which can reduce Au3+, and subsequently be oxidized into their carboxylic acid form. The present invention modifies and optimizes such process through microwave-assisted heating to control the molecular weight size range of the final hydrolyzed starch which is shown to affect the resulting size and distribution of the AuNP. Microwave reactor was used to control the temperature of the starch hydrolysis and the resulting solutions were characterized for its Z-average particle size (PS) using dynamic light scattering (DLS) and weight-average molecular weight (MW) using the same instrument in static light scattering (SLS) mode. The Z-average particle size is also called harmonic intensity-weighted arithmetic average particle diameter, whereas the weight-average MW is derived from the Debye plot of the scattering intensity of the starch solution with respect to its concentration.
Optimization aims to maximize the yield, minimize the size, and maximize the stability of the resulting AuNP ink from the synthesis process in this invention. To do this, the amount of the reducing species from the hydrolyzed starch was varied by changing the microwave-assisted hydrolysis conditions, i.e., temperature (70, 80, 90° C.) and time (15, 30, 45 min), using a microwave reactor. By maximizing the yield and minimizing the size of the gold nanoparticles, the final loading of the AuNP in the ink will be greater. This can contribute to increased conductivity of the printed gold patterns due to tight-packing of smaller particles. The ratio of starch-to-gold is also further optimized to increase the concentration of AuNP in the ink formulation for the target printed electronics application.
The microwave-hydrolyzed starch samples obtained from the process previously described are used to synthesize AuNP using the process as described in the examples; typically mixing equal volumes of the hydrolyzed starch (4% w/v) and an AuNP precursor solution prepared from 0.1 M HAuCl4 solution, adjusted to pH 7 by NaOH and then heating again in the microwave reactor for 5 min. Here, the concentration of the precursor Au3+ in the final reaction mixture may be fixed to 25.0 mM, which is a rather high concentration compared with other reported synthesis processs, typically 1.0-12.5 mM.
The resulting AuNP solutions from the examples were characterized by UV-Vis spectroscopy (
Increased starch hydrolysis temperature from step 1, e.g., 70° C. to 90° C. resulted in blue-shifted SPR peak of the synthesized AuNP from 550 to 530 nm (
The trend in particle size observed in the UV-Vis spectra was also confirmed with the Z-average particle size measurement using DLS, where there was a decrease in the particle size of the AuNP-starch nanoparticles from about 175 nm to about 50 nm with increased heating temperature from 70° C. to 90° C. (
Another value derived from the cumulants analysis of the DLS data is the polydispersity index (PdI), which is a dimensionless number indicating the broadness of the size distribution around Z-average size. It ranges from 0.0 to 1.0 with 1.0 being the most polydisperse. In the case of the synthesized AuNP samples in this study, PdI values range from 0.07-0.25, which indicate relatively narrow size distributions of AuNP. No obvious trend was observed for PdI value with respect to the starch hydrolysis time and temperature (
Zeta potentials of the AuNP samples were also measured (
It is also noted that the starch-to-gold ratio is quite low in the synthesized AuNP dispersion, which is less than 2 wt % (
The synthesized AuNP ink obtained from the process described above can be printed using a drop-on-demand (DoD) inkjet printer. As previously mentioned, the jettability of an ink can be predicted by gauging its fluid properties such as viscosity, surface tension, and density.
For the ink obtained from the above-discussed process and the examples below, viscosity is approximately measured to be 1.20 cP, surface tension is 69.7 mN/m, and the density is 1.0246 g/mL; using Eq. 1, Z-number for this ink is 54.6. This is beyond the range set by most studies on inkjet printing. However, as discussed and demonstrated by Subramanian et al., there are inconsistencies regarding jettability criteria using Z-number [Nallan, H. C.; Sadie, J. A.; Kitsomboonloha, R.; Volkman, S. K.; Subramanian, V. Langmuir 2014, 30, 13470-13477.]. In any case, the resulting ink was successfully jetted to single drops without satellite droplets as shown in the
Small angle X-ray scattering (SAXS) in transmission mode was done to measure the particle size in the ink (
Inkjet-printed films obtained using the ink from the above-described process, as well as the examples, were characterized for their electrical property by measuring the sheet resistance upon drying and heating to reach the sintering temperature. The trend in conductivity (
The morphology of the printed gold films heated at 50° C. and 400° C. was observed using AFM and FE-SEM (
After heating at 400° C., starch would be degraded and particles coalesce further together, forming a network of gold aggregates. This is observed in both the AFM and SEM images where bigger particles are mostly present in fused network and are seen embedded in the aggregates. Aside from the increase in particle size, it can also be observed that particle boundaries became less sharp in terms of contrast due to coalescence at these boundaries.
It was also observed that the starch (<2%) on the gold is burned off and not detectable by surface enhance infrared spectroscopy (SEIRA). The samples for FT-IR were prepared by dropping a small volume of the AuNP on KBr powder which was then dried, heated at the indicated temperature in
The following representative examples of performing the process according to the present invention are for purposes of illustration only and are not meant to limit the invention in any way.
Typically, a 4% (w/v) solution of starch (e.g., potato starch from Sigma-Aldrich) was prepared by dissolving the starch in distilled water with gentle heating and stirring either using a hot plate or a kitchen microwave oven with 5-s bursts. This is allowed to cool down to room temperature prior to hydrolysis. For the hydrolysis mixture: equal volumes of the 4% starch solution is mixed with 0.1 M NaOH solution. Using a laboratory microwave reactor (e.g., Milestone flexiWAVE) a temperature program is inputted making sure that the heating is optimized for the volume of solution to be heated. Typically, for a 60-mL reaction volume, the microwave reactor parameters are: 30% stirring speed, 2-min ramp time to target temperature (90° C.), 500 W ramp-power and hold-time of 60 min at 200 W hold-time power. This program yields a fine control of the temperature of the reaction mixture which should aid in producing the optimized sizes of the hydrolyzed starch. This is cooled down to room temperature prior to use in the AuNP synthesis.
The Au3+ precursor solution is a “10% Au precursor” which may be prepared from a gold salt (AuHCl4) or starting from the pure metal. Starting from the pure metal, 10 g of gold pellets (>98% purity) is dissolved with 60 mL of aqua regia solution which is made up of 1 HNO3 (r.g.): 3 HCl (r.g.) mixture, done in the fume hood. This mixture in a beaker is heated on a hotplate with stirring at about 80° C. to aid in the dissolution. This step generates brown, toxic NOx gas and so a fume hood is necessary. When the solid gold has completely dissolved, the liquid is allowed to evaporate until the residue almost dry (not completely dry). This residue is then dissolved in 100 mL with deionized (DI) water to produce the “10% Au precursor”.
A 2% Au3+ solution with pH 7.0 is prepared from an aliquot of the 10% stock solution wherein a 50% NaOH solution was used to neutralize the aliquot prior to final dilution with distilled, deionized water to make the 2% Au3+ final concentration.
Next, equal volumes of the 2% Au3+ solution and 0.1 M NaOH are mixed. Using this, and an equal volume of the hydrolyzed starch solution (a freshly hydrolyzed sample as much as possible) is mixed together in a microwave vessel—for a 60-mL reaction mixture, 30 mL of each is mixed. This mixture is then heated uniformly using an optimized microwave heating program to control the temperature profile. For a 60-mL reaction mixture using a Milestone flexiWAVE microwave reactor, the parameters are: 30% stirring speed, 1-min ramp time to reach the target 60° C. temperature at 500 W ramp power, and 15 min hold time at 100 W. This should produce the AuNP solution which is then allowed to cool to room temperature before transferring to a clean container.
To prepare the ink, the AuNP is washed thoroughly with water to remove residues from the microwave synthesis reaction. This was achieved by repeated (at least 2×) washing with distilled, deionized (DI) water with sonication and centrifugation at 13,000 rpm for 15 min. First, the reaction mixture is centrifuged, and the supernatant discarded. The AuNP pellet is dispersed in fresh DI water with ultrasonication at 500 W for 15 min. This is centrifuged again at 13,000 rpm for 15 mins, discarding the supernatant, and fresh DI water added to repeat the washing step above.
Finally, the gold ink is prepared from the washed AuNP, as a concentrated dispersion which is about 1/20th of the original volume from the microwave synthesis reaction step. This AuNP ink is stored in the refrigerator (ca. 4° C.). It remains stable for more than a month.
Prepared AuNP inks were filtered first through 0.45 μm membrane prior to loading in an inkjet printer cartridge or ink container. Here, a Jetlab® 4 from MicroFab Technologies, Inc. is used to demonstrate jettability and printability of the ink formulation. A customized bipolar waveform (
To minimize the coffee ring effect on the printed film, Triton X-100 is added to the ink formulation to make about 0.3% (w/w). This results in printed gold lines with enhanced fidelity of the print (
The example illustrates an embodiment of the present invention. Alternative embodiments will suggest themselves to those skilled in the art in light of the above disclosure. It is to be understood, therefore, that changes may be made in the particular embodiments described above which are within the full intended scope of the invention as defined in the appended claims.
Number | Date | Country | Kind |
---|---|---|---|
12019000076 | Feb 2019 | PH | national |
Filing Document | Filing Date | Country | Kind |
---|---|---|---|
PCT/PH2020/050001 | 2/7/2020 | WO | 00 |