Abstract
The interaction of methylene blue and crystal violet dyes with a range of gold nanoparticles (AuNPs), gold nanoclusters and gold/silver nanoclusters is reported. It is found that 20 nm citrate-capped AuNPs have strong interactions with these two dyes that result in red-shifted absorption peaks in their electronic absorption spectra. Transmission electron microscopy and dynamic light scattering measurements show that this can be attributed to these AuNPs combining into large agglomerates. Eventually, precipitation is observed. The agglomeration process is triggered when the dye reaches or exceeds a threshold concentration and then does not stop until all the AuNPs have agglomerated into large particles and precipitated. Calculations suggest that the threshold concentration corresponds to having sufficient dye molecules to form a monolayer on the surface of AuNPs. We also observe similar red-shifting in the absorption peaks of the electronic absorption spectra of 11–50 nm citrate-capped AuNPs formed by both single step and seeded growth methods. No such interactions were observed in the UV-vis spectra of the dyes with Tris-capped AuNPs, gold nanoclusters or gold/silver nanoclusters.
Highlights
There is a great deal of interest in improving our understanding of the interactions between metal nanostructures and adsorbate molecules
We have shown that the interaction previously observed between cationic dyes and 11 nm citrate-capped AuNPs11,12 is found in other citrate-capped AuNPs
For Tris-capped NPs and cysteineprotected NCs, we did not observe any new features in the UVvis spectra
Summary
There is a great deal of interest in improving our understanding of the interactions between metal nanostructures and adsorbate molecules. One particular example is the interaction between gold nanoparticles (AuNPs) and photosensitiser dyes. The interaction between CV and gold nanoclusters (AuNCs) has a similar effect,[5] AuNCs seem to alter the pathway of the dye sensitization process and speci cally generate hydrogen peroxide[6] rather than the mixture of ROS that is seen for dye–AuNP systems. In order to understand how combining dyes, such as CV or MB, with AuNPs and AuNCs enhances the generation of ROS, it is important to improve our understanding of the dye–gold interaction. There are numerous reports of morphological changes when AuNPs are mixed with cationic dyes, with the AuNPs undergoing agglomeration to form large clusters.[7,8,13]
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