Recyclable colorimetric sensor of Cr^{3 +} and Pb^{2 +} ions simultaneously using a zwitterionic amino acid modified gold nanoparticles

In this work, a rapid, simple and sensitive colorimetric sensor for simultaneous (or respective) detection of Cr^{3 +} and Pb^{2 +} using tyrosine functionalized gold nanoparticles (AuNPs^Tyr) has been developed. Tyrosine, a natural and zwitterionic amino acid, could be as a reducing and capping agent to synthesise AuNPs and allow for the simultaneous and selective detection of Cr^{3 +} and Pb^{2 +}. Upon the addition of Cr^{3 +} or Pb^{2 +} (a combination of them), the color of AuNPs^Tyr solution changes from red to blue grey and the characteristic surface plasmon resonance (SPR) band is red-shifted to 580 nm due to the aggregation of AuNPs. Interestingly, the aggregated AuNPs^Tyr can be regnerated and recycled by removing Pb^{2 +} and Cr^{3 +}. Even after 3 rounds, AuNPs^Tyr show almost the same A_{580 nm} / A_{520 nm} value for the assays of Pb^{2 +} and Cr^{3 +}, indicating the good recyclability of the colorimetric sensor. The responding time (within 1 min) and sensitivity of the colorimetric sensor are largely improved after the addition of 0.1 M NaCl. Moreover, the AuNPs^Tyr aggregated by Cr^{3 +} or Pb^{2 +} (a combination of them) show excellent selectivity compared to other metal ions (Cr^{3 +}, Pb^{2 +}, Fe^{2 +},Cu^{2 +},Zn^{2 +},Cr^{6 +},Ni^{2 +},Co^{2 +},Hg^{2 +},Mn^{2 +},Mg^{2 +},Ca^{2 +},Cd^{2 +}). More importantly, the developed sensor manifests good stability at room temperature for 3 months, which has been successfully used to determine Cr^{3 +} and Pb^{2 +} in the real water samples with a high sensitivity.