Smartphone-based disposable cotton-swab sensor loaded with creatinine/Cu–chlorophyllin–stabilized AuNPs for ultrasensitive RGB colorimetric detection of mercury (II) ions in aquatic environments

Abstract

A simple, rapid, selective, and sensitive colorimetric sensor based on a disposable cotton swab loaded with Au nanoparticle–modified Cu-chlorophyllin (Au0–NPsCHL) and creatinine was developed for mercury (II) detection in aquatic environments using smartphone–based RGB analysis. Au0–NPsCHL were synthesized by ultrasonic-assisted chemical reduction, employing Cu-chlorophyllin as a stabilizing and NaBH4 as a reducing agent. A vivid red Au0–NPsCHL colloidal (7.96 ± 0.29 nm) with a sharp SPR peak at 515 nm was successfully obtained. An aliquot of 75 µL of Au0-NPsCHL (0.103 ± 0.03 nmol L−1) and 25 µL of creatinine solution (40 mg L−1) were sequentially loaded into an 8.0 × 0.5 cm cotton swab, separated by a 0.5 cm air gap. Detection began by immersing the swab into Hg2+–contaminated samples for 3 min, allowing Hg2+ adsorption. Breaking the swab generated pressure differential, which, along with gravity, drove the Au0–NPsCHL to mix with creatinine. The resulting mixture migrated toward the swab tip by capillary action and reacted with adsorbed Hg2+ through metallophilic 5d10–5d10 interactions. Creatinine acted as a bridging ligand, inducing Au0–NPsCHL aggregation and shifting the SPR to 620 nm, causing a visible red-to-blue transition. This sensor enables rapid and visual detection and quantitative evaluation via smartphone RGB analysis. The system demonstrated excellent linearity from 1 to 100 µg L−1 with a low LOD of 0.82 µg L−1. Recoveries of 97.6–101.8 % confirmed high accuracy and minimal matrix interference. The disposable cotton swab RGB sensor provides a rapid, portable, and practical tool for on-site primary assessment of Hg2+ contamination in environmental samples.