Photoluminescence measurements of carbon quantum dots within three-dimensional hydrogel matrices using a high throughput 96 well plate method

Solid or liquid platforms have been traditionally employed for measuring the fluorescent properties of quantum carbon dots (QCD). Hydrogels possess both liquid and solid properties which allow them to overcome several shortfalls of both solid and liquid sensing platforms. Hydrogels offer a three dimensional platform which can house nanoparticles with different attributes (i.e. fluorescent QCD's) and prevents their aggregation. Here, we incorporate QCD's (made from the hydrothermal treatment of 1-naphthylamine and citric acid) into the matrix of a zinc oxide hydrogel. This nanocomposite was shown to have hexavalent chromium (Cr⁶⁺) specific fluorescence quenching properties. Detailed fluorescence analysis of the hydrogel with Cr⁶⁺ was conducted in a high throughput manner by loading the hydrogel into wells of a black 96-well plate. Fluorescence quenching of the hydrogel-QCD-nanocomposites in the presence of dilutions of Cr⁶⁺ was measured using a fluorescence spectrophotometer and showed incremental fluorescence decreases with increasing Cr⁶⁺ concentration. Furthermore, this was quantitatively confirmed by Stern-Volmer plots showing a linear quenching trend (R² = 0.9975) when comparing fluorescence intensities against increasing Cr⁶⁺ concentrations (0.234-1.875 μM). This technology can be applied for routine water quality testing in agricultural, natural and potable water sources for the early detection of heavy metal pollutants.