A novel, highly sensitive and selective dual-readout (colorimetric and fluorometric) sensor based on fluorescent carbon dots (CDs) and unmodified gold nanoparticles (AuNPs) for the detection of thiocyanate (SCN(-)) was proposed. Amino-functionalized CDs could be readily adsorbed onto the surface of citrate-stabilized AuNPs through Au-N interactions, leading to the aggregation of AuNPs and the nonfluorescent off-state of CDs arising from potential fluorescence resonance energy transfer (FRET). However, SCN(-) had a stronger affinity toward AuNPs and could compete with CDs to bind onto the surface of AuNPs in priority, which prevented the aggregation of AuNPs and fluorescence quenching of CDs. Correspondingly, both the colorimetric and fluorometric signals remained "light-on". The color of the sensing solution remained red and the fluorescence remained unquenched. A distinguishable change in the color was observed at a SCN(-) concentration of 1 μM by the naked eye and a detection limit as low as 0.036 μM was obtained by virtue of fluorescence spectroscopy. Both colorimetric and fluorometric sensors exhibited excellent selectivity toward SCN(-) over other common metallic ions and anions. In addition, such a sensing assay featured simplicity, rapidity, cost-effectiveness and ease of operation without further modification. The accuracy and precision were evaluated based on the quantitative detection of SCN(-) in tap water and saliva samples with satisfactory results.
A simple colorimetric assay for phosphate ion (Pi) has been established based on analyte-induced inhibition of the magnetite nanoparticles (MNPs)-catalyzed oxidation of 3,3 0 ,5,5 0 -tetramethylbenzidine (TMB) in the presence of H 2 O 2 . The Fe 3 O 4 MNPs can catalyze the H 2 O 2 -mediated oxidation of TMB and yields a blue oxidized product, which exhibits a maximum absorption at 652 nm. Pi could be absorbed on the surface of the Fe 3 O 4 MNPs through coordinating with Fe 3+ , inducing a reduced colorimetric signal. The colorimetric signal change (DA 652 ) in this process was proportional to the concentration of Pi, ranging from 0.2 mM to 200 mM. The limit of detection (S/N ¼ 3) was as low as 0.11 mM. The as-proposed Fe 3 O 4MNPs-TMB-H 2 O 2 probe exhibited a high selectivity toward Pi over other relevant ions that commonly exist in water and has been applied to Pi detection in drinking water, ground water and lake water samples with satisfactory results.
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