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Chitosan-grafted thymol (CST) coated on gold nanoparticles has been synthesized and characterized for the design of antimicrobial materials. CST was synthesized via adapting the Mannich reaction, and it acted as the capping agent for the synthesis of gold nanoparticles (AuNPs). The grafting of thymol onto the side chain of chitosan has provided a degree of substitution value (%DS NMR ) of 10.0%, calculated by nuclear magnetic resonance spectroscopy. UV–visible spectrometry and elemental analysis were used to confirm the successful synthesis of CST through adapting the Mannich reaction. The appropriate concentration of CST for AuNP synthesis was found to be 0.020%w/v. A red-wine colloidal AuNP solution of 2.41–3.30 nM particle size exhibits a strong surface plasmon resonance at 502 nm, which shows negative charges at pH = 9 of −36.37 mV. This result evidenced that the AuNPs showed electrostatic repulsion and CST played a role as a capping agent to provide a good dispersion and stability state. CST coated on the AuNP surface was successfully utilized for the control of cariogenic bacteria in the oral cavity. The results obtained from this study show that the tuning of the capping agent used in the synthesis step strongly influences the latter antimicrobial activity of the nanoparticles against Streptococcus mutans ATCC 25175 and Streptococcus sobrinus ATCC 33402 activity, with an inhibition zone of 15.90 and 14.25 mm, respectively. The average minimum inhibitory concentration values against S. mutans ATCC 25175 and S. sobrinus ATCC 33402 were found to be 25 and 100 mg/L, respectively, whereas the minimum bactericidal concentration values were 100 and 200 mg/L, respectively.

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A new colorimetric sensor based on gold/silver bimetallic nanoparticles (Au–Ag BNPs) for the sensitive and selective detection of mercury (II) was developed. Gold nanoparticles (AuNPs) were synthesized by Turkevich method. The surface modification of AuNPs was modified by the layer–by–layer technique using poly(diallyl dimethylammonium chloride) which provided positively charged of AuNPs. Negatively charged silver nanoparticles (AgNPs) were synthesized by chemical reduction using poly(4–styrenesulfonic acid–co–maleic acid) as the stabilizing agent. The layer–by–layer assembly deposition technique was used to prepare Au–Ag BNPs of positively and negatively charged of AuNPs and AgNPs, respectively. The synthesized Au–Ag BNPs were characterized by a UV-visible spectrophotometer, zeta potential analyzer, FT–IR, TEM, XRD, and EDX. The Au–Ag BNPs sensor was able to detect mercury (II) in aqueous solution, visibly changing from brownish–orange to purple. The linear relationships of the UV-visible spectrometry demonstrate that the Au–Ag BNPs-based colorimetric sensor can be used for the quantitative analysis of mercury (II) in the range of 0.5–80 mg L−1, with the correlation coefficient, r2 = 0.9818. The limit of detection (LOD) of mercury (II) was found to be 0.526 + 0.001 mg L−1. The BNPs is also verified to have a good practical applicability for mercury (II) detection in the real samples.