Smartphone-assisted cost-effective approach for detecting mercury(II) using papain stabilized fluorescent gold nanoclusters

The abnormal accumulation of mercury (Hg2+) ions in the human body can cause serious health problems. Therefore, there is growing interest on developing fluorescence-based methods for the facile, cost-effective and real-time detection of Hg2+ ions in environmental samples. Herein, red-emitting papain-stabilized gold nanoclusters (pap-AuNCs) were post-functionalized with salicylaldehyde (SA) and employed for the detection of Hg2+ ions. When SA was added to pap-AuNCs (λex = 380 nm, λem = 670 nm), an imine linkage was formed between the free –NH2 group of papain functionalized over AuNCs and –CHO group of SA, resulting in the formation of orange-emitting SA_pap-AuNCs (λem = 498 nm, 670 nm). Upon the addition of Hg2+, the modified SA_pap-AuNCs showed an instantaneous and selective fluorescence quenching at 670 nm due to the metallophilic Hg2+-Au+ interactions without altering the fluorescence intensity at 498 nm, which led to the fluorescent colour change from orange to green. The fluorescent colour change of SA_pap-AuNCs was integrated with a smartphone for the on-site cost-effective detection of Hg2+. Without any interference from other metal ions, the fluorescence spectral and colour responses from SA_pap-AuNCs can be used to detect Hg2+ ions down to 0.43 µM and 3.74 µM, respectively. Finally, the practical application of SA_pap-AuNCs was validated by quantifying Hg2+ in real environmental water samples.