Luminescent functional materials based on nanoclusters (NCs) have unique optical properties. In an article recently published in ACS Applied Nano Materials, researchers discussed the behavior of gold nanoclusters (AuNCs) on modifying with 6-propyl-2-thiouracil (PRT) and trimesic acid (TA). This coassembly resulted in the formation of PRT-AuNCs/TA complex-based microrods with enhanced photoluminescence emission properties.
Study: Au Nanocluster-Based Smart Multicolor Luminescent Hydrogels for Encryption Applications. Image Credit: wk1003mike/Shutterstock.com
An ordered arrangement of the coassembled PRT-AuNCs/TA aggregates was observed due to the (pie-pie) π–π stacking and intermolecular hydrogen bonding between PRT and TA. Additionally, these interactions improved the luminescence in PRT-AuNCs.
Due to the excellent optical properties of the PRT-AuNCs/TA aggregates, the researchers loaded them into the network of κ-carrageenan to prepare a luminescent nanocomposite hydrogel which exhibited high strength, strong luminescence, and self-healing properties. Additionally, pH-responsive multicolored hydrogels were prepared by doping PRT-AuNCs with blue emission quantum dots (QDs) with applications in information encryption.
The three-dimensional (3D) network of hydrogels enables the trapping of high water content inside their network. Additionally, these hydrogel materials have highly desirable properties like the ability of self-healing, responsiveness to external stimuli, and transparent appearance.
Due to the extraordinary optical properties and potential applications of luminescent hydrogels, they have received considerable attention from researchers. The conventional method of preparing this type of hydrogels involved the incorporation of QDs and organic dyes into a gelation matrix. However, this preparation method resulted in luminescent hydrogels with a lifetime as short as nanosecond level. Additionally, the presence of dye molecules in these hydrogels results in aggregation-induced quenching (ACQ) of fluorescence.
These limitations of luminescent hydrogels can be overcome by doping inorganic−organic hybrid materials due to multiple advantages such as the feasibility of structural decoration, excellent biological performance accompanied by ultrasmall size, and versatile luminescence properties.
AuNCs and inorganic−organic hybrid material (except lanthanide coordination compounds), received considerable attention from researchers due to their multiple advantages, such as ultrasmall size and consequent prominent biological properties, versatile luminescence properties, and structural decoration feasibility. These advantages promote the application of AuNC-based luminescent materials in sensors, data encryption, and light-emitting displays.
To date, the luminescent hydrogels were developed with only one luminous center. However, incorporating multiple kinds of luminogens into the same hydrogel matrix could produce hydrogels with multicolor centers. Luminogens result in emission intensity and pH-responsive color changes that could improve the accuracy of the sensors used in information security systems.
AuNCs-based Smart Multicolor Luminescent Hydrogels
In the present study, the researchers synthesized PRT-AuNCs by protecting the AuNCs with PRT. The as-prepared PRT-AuNCs had adequate water solubility and hydrogen bonding sites. The coassembly of PRT-AuNC solution with TA resulted in the luminescent hydrogel with improved photoluminescence.
The synthesized PRT-AuNCs were characterized using various analytical methods. The ultraviolet-visible (UV-vis) spectroscopy showed a new absorption band at 304 nanometers absent in PRT spectra, indicating the formation of PRT-modified AuNCs. The Fourier transform infrared (FTIR) spectra showed a peak at 1638-centimeter inverse, which corroborated the carbonyl groups of PRT, confirming the surface modification of AuNCs with PRT ligands. The high-resolution transmission electron microscope (HR-TEM) showed that the PRT-AuNCs were in the form of well-dispersed nanoparticles.
The π−π stacking interactions and hydrogen bonding between TA and PRT ligands resulted in micrometer-scale rod-like aggregates with strong emissions. Furthermore, incorporating the synthesized luminescent microrods into polysaccharide κ-carrageenan resulted in a nanocomposite hydrogel with excellent photoluminescent and high strength properties. Additionally, the polymer networks with hydrogen bonds in the nanocomposite hydrogel resulted in their superior self-healing capacity.
Loading blue emission QDs into the above-mentioned nanocomposite hydrogel induced a multicolor emission with a color change from orange to blue, regulated by the surrounding pH. The concurrent properties, including pH responsiveness of PRT-AuNCs and QDs and the self-healing properties of prepared the nanocomposite hydrogel made it a promising candidate for application in information encryption.
Along with the improved luminescent property of PRT-AuNCs, this study demonstrated the application of the synthesized NCs in the preparation of anticounterfeiting materials and luminescent hydrogels.
In summary, the researchers prepared a luminescent hydrogel by incorporating AuNCs-based luminogens into the κ-carrageenan gelation matrix. The luminescent property of hydrogel was improved by coassembling PRT-AuNCs with TA. The π–π stacking and intermolecular hydrogen bonding between PRT and TA were the driving forces that facilitated the formation of ordered microrods with strong photoluminescence properties.
A luminescent nanocomposite hydrogel was prepared by loading the PRT-AuNCs/TA complex into κ-carrageenan gelation networks. Furthermore, doping blue emission QDs into the composite gel system resulted in a multicolor hydrogel with a luminescent color change from orange to blue, regulated by surrounding pH that exhibited its promising application in information encryption.
With the help of this work, the team improved the PRT-AuNC’s luminescent properties through their coassembly with TA, which helped broaden its applications in preparing anticounterfeiting materials and luminescent hydrogels.
Yu, Y., Zhang, X., Wang, M., Luan, J., Liu, X., Shen, J and Qi, W. (2022) Au Nanocluster-Based Smart Multicolor Luminescent Hydrogels for Encryption Applications. ACS Applied Nano Materials. https://pubs.acs.org/doi/10.1021/acsanm.2c02496