Synthesis of quantum dot-based fluorescent sensors for detection of anionic metabolites and copper (ii) ion
Authors:   , Phromsiri Nattakarn 
Published: September 2023

Oxalate, citrate, and urate are the anionic metabolites that are the products of the metabolism pathway of living organisms. The high level of these anionic metabolites is related to the diseases. For example, the excess amount of oxalate ions in urine indicates the risk of kidney stones, while the level of citrate can suggest the risk of cancer. The level of urate in urine is related to the risk of gout. The detection of these anionic metabolites is significant and still remains a challenge. Thus, this work aims to develop the more sensitive and selective fluorescence sensing of oxalate in aqueous-based by using dinuclear copper(II) complex, Cu2L with eosin Y dyes that was reported as a fluorescence sensing for oxalate in the water earlier with quantum dots (QDs). However this complex shows a higher sensitivity to urate rather than oxalate. To improve the sensitivity and selectivity of the sensing system, the ratiometric and fluorescence resonance energy transfer (FRET) was applied to the new approach. To fabricate the ratiometric and FRET system of eosin Y to QDs, the blue-emitting QDs are required. Unfortunately, the blue-emitting aqueous-based QDs are mostly quenched by Cu2+ ions. Hence, it has become the two fluorescence sensing projects which are the detection of urate and Cu2+ ions.  The first naked-eyes probe called the “dual-dyes probe” was constructed for urate detection. According to the ratiometric fluorescence method and FRET, this dual-dyes system has an indicator displacement assay (IDA) for the fluorescence turn-on mechanism. Blue-emitting quinine sulfate dyes is a donor and green-emitting eosin Y is an acceptor in the FRET system. The energy transfer was confirmed by fluorescence titration. Eosin Y also acts as an indicator in IDA with the Cu2L as a host. Dual-dyes probe responds to 3 main anions which are urate, oxalate, and citrate. The detection limit are 0.0699, 0.3790, and 1.0472 µmol L-1, respectively. According to the urate, oxalate, and citrate anions concentration found in the urine are 1.49-4.46, 0.7-2.3, and 0.13-0.46 mmol L-1 [1, 2]. With the optimal dilution factor, we are able to obtain the selective probe for urate detection and it can be operated in the synthetic urine and gout patient's mimic synthetic urine samples. This approach is simple, convenient, fast, sensitive, selective, and low-cost without any pretreatment step and provides the obvious fluorescence color changing from blue to green in the presence of urate. This obvious change in the fluorescence color would lead to the opportunity of the onsite test kit for the preliminary screen of gout. The second probe is a mixed-QDs probe for the detection of Cu2+. Although, the Cu2+ ion is one of the essential ions for humans, the excessive amount of it is toxic. Additionally, Cu2+ is a toxic heavy metal in the environment. The facile, simple, sensitive, and selective probe for Cu2+ detection was fabricated by using the ratiometric system and FRET between 2 different types of QDs. There are blue-emitting Si QDs (donor) and green-emitting CdSe QDs (acceptor). The mixed-QDs probe has a yellow-green fluorescence color. The FRET between Si QDs and CdSe QDs was confirmed by the fluorescence titration and time-resolved fluorescence. The fluorescence color chang from the yellow-green of the mixed-QDs to the blue-emission of the only Si QDs was observed in the addition of Cu2+. The X-ray photoelectron spectrometer (XPS) technique was used to study the effect of Cu2+ on to QDs surfaces. The change mainly depends on the quenching of CdSe QDs surfaces. The mixed-QDs probe shows a very low detection limit of 3.89 nmol L-1 and has a high selectivity toward Cu2+ ions compared to Co2+, Fe3+, Zn2+, Al3+, Mg2+, Cr3+, Ba2+, Li+, Ca2+, Sr2+, Ag+, Na+,  Ni2+, K+, Cd2+, Pb2+, Mn2+, and Hg2+ions.