Effective GQD/AuNPs nanosensors for selectively bifunctional detection of lysine and cysteine under different photophysical properties

[Display omitted] •The AuNPs/GQDs nanocomposite has been successfully prepared by one-pot chemical reaction using GQDs as reducing agent.•GQDs performs an effective reducing agent and stabilizer.•AuNPs/GQDs served as highly selective detection of cysteine (Cys) by UV–vis change and lysine (Lys) by turn-on fluorescence undergone the aggregation process and the displacement process, respectively, in PBS pH 7.4.•The use of AuNPs/GQDs as the chemosensor highlighted a good quantitative determination of Cys in urine samples and Lys in milk samples with real time analysis. This work presented the development of the optical nanosensor based on AuNPs and GQDs which perform the efficiency dual optical sensor. Importantly, the AuNPs with GQDs was prepared by one-pot chemical reaction by using GQDs as an excellent bifunction of the reducing agent and stabilizer to obtain the disaggregated AuNPs/GQDs in red color property. Apart from its photophysical sensing ability, this hybrid material demonstrated the dual sensing of cysteine and lysine regarding to colorimetric and fluorometric techniques, respectively, in PBS solution pH 7.4. The presence of Cys in the AuNPs/GQDs solution rapidly lead to the color change from red to blue with concomitant of the red shift of the absorption band at 525 to 645 nm undergone the aggregation process. On the other hand, AuNPs/GQDs nanosensor in the presence of Lys performs the off-on fluorescence at 450 nm without color change in PBS solution pH 7.4. Interestingly, no interference of any amino acids towards Cys and Lys detection highlighted a high efficacy in selective sensing aspect. For sensing application, the detection limit of Cys and Lys was 5.88 μM and 16.14 μM, respectively, with an acceptable % recovery of 101–106 in real samples.