Biothiol modulated growth and aggregation of gold nanoparticles and their determination in biological fluids using digital photometry

O-Phthalaldehyde Derivatization for the Paper-Based Fluorometric Determination of Glutathione in Nutritional Supplements

Herein, a new, direct paper-based fluorimetric method is described for the quantitative determination of glutathione (GSH) molecules in nutritional supplements. Briefly, the proposed analytical method is based on the fluorescence emission resulting from the direct and selective chemical reaction of GSH molecules with the derivatization reagent that is o-phthalaldehyde (OPA) in acidic conditions at room temperature. The intensity of the emitted fluorescence on the surface of the analytical paper devices after irradiation with a lamp at 365 nm is proportional to the concentration of GSH and is measured using a smartphone as the detector. This methodology, which is suitable for measurements in laboratories with limited resources, does not require specialized instrumentation or trained personnel. The protocol governing the proposed method is simple and easily applicable. Essentially, the chemical analyst should adjust the value of pH on the surface of the paper by adding a minimal amount of buffer solution; then, after adding a few microliters of the derivatization reagent, wait for the surface of the paper to dry and, finally, add the analyte. Subsequently, the irradiation of the sensor and the measurement of the emitted fluorescence can be recorded with a mobile phone. In the present study, several parameters affecting the chemical reaction and the emitted fluorescence were optimized, the effect of interfering compounds that may be present in dietary supplements was examined, and the stability of these paper sensors under different storage conditions was evaluated. Additionally, the chemical stability of these paper devices in various maintenance conditions was studied, with satisfactory results. The detection limit calculated as 3.3 S/N was 20.5 μmol L-1, while the precision of the method was satisfactory, ranging from 3.1% (intra-day) to 7.3% (inter-day). Finally, the method was successfully applied to three different samples of dietary supplements.

Use of metal nanoparticles for preconcentration and analysis of biological thiols

Metal nanoparticles (NPs) exhibit several unique physicochemical properties, including redox activity, surface plasmon resonance, ability to quench fluorescence, biocompatibility, or a high surface-to-volume ratio. They are being increasingly used in analysis and preconcentration of thiol containing compounds, because they are able to spontaneously form a stable Au/Ag/Cu-S dative bond. They thus find wide application in environmental and particularly in medical science, especially in the analysis of biological thiols, the endogenous compounds that play a significant role in many biological systems. In this review article, we provide an overview of various types of NPs that have been applied in analysis and preconcentration of biological thiols, mainly in human biological fluids. We first discuss shortly the types of NPs and their synthesis, properties, and their ability to interact with thiol compounds. Then we outline the sample preconcentration and analysis methods that were used for this purpose with special emphasis on optical, electrochemical, and separation techniques.

Analyte-mediated formation and growth of nanoparticles for the development of chemical sensors and biosensors

The cornerstone of nanomaterial-based sensing systems is the synthesis of nanoparticles with appropriate surface functionalization that ensures their stability and determines their reactivity with organic or inorganic analytes. To accomplish these requirements, various compounds are used as additives or growth factors to regulate the properties of the synthesized nanoparticles and their reactivity with the target analytes. A different rationale is to use the target analytes as additives or growth agents to control the formation and properties of nanoparticles. The main difference is that the analyte recognition event occurs before or during the formation of nanoparticles and it is based on the reactivity of the analytes with the precursor materials of the nanoparticles (e.g., metal ions, reducing agents, and coatings). The transition from the ionic (or molecular) state of the precursor materials to ordered nanostructured assemblies is used for sensing and signal transduction for the qualitative detection and the quantitative determination of the target analytes, respectively. This review focuses on assays that are based on analyte-mediated regulation of nanoparticles' formation and differentiate them from standard nanoparticle-based assays which rely on pre-synthesized nanoparticles. Firstly, the principles of analyte-mediated nanomaterial sensors are described and then they are discussed with emphasis on the sensing strategies, the signal transduction mechanisms, and their applications. Finally, the main advantages, as well as the limitations of this approach, are discussed and compared with assays that rely on pre-synthesized nanoparticles in order to highlight the major advances accomplished with this type of nano-sensors and elucidate challenges and opportunities for further evolving new nano-sensing strategies.

Gold-activated luminol chemiluminescence for the selective determination of cysteine over homocysteine and glutathione

This work reports a chemiluminescence assay for the highly selective determination of cysteine in biological fluids without separation techniques. The method is based on the ability of cysteine to selectively enhance the metal-catalyzed chemiluminescence generated by the oxidation of luminol from gold tetrachloride anions under alkaline conditions. The selectivity of the method stems from the fact that, under strongly alkaline conditions, the formation of the four-membered ring transition state of cysteine is less favorable as compared to the formation of the respective 5- and 9- membered ring transition states of homocysteine and glutathione, respectively. These transition states exert stronger hindrance and hydrophobic interactions repelling the negatively charged luminol dianion and possibly exhibit lower reducing ability for dissolved oxygen, towards the formation of superoxide radicals, thus reducing the oxidation of luminol. Under the optimum experimental conditions, the linear range of the method extended from 0.5 to 20 μΜ while cysteine could be determined at concentrations as low as 0.5 μM, with good reproducibility (<3.5%) and recoveries between 80 and 93% in artificial and real biological fluids.

Metal nanoparticles based lab-on-paper for phenolic compounds evaluation with no sample pretreatment. Application to extra virgin olive oil samples

In this work, a low-cost, disposable, and portable lab-on-paper device is proposed to simultaneously quantify total polyphenol content (TPC) and antioxidant capacity (AOC) in 15 min; the assay requires no pre-treatment of the samples. The lab-on-paper device fabrication has been carried out employing a xurography-based benchtop microfabrication technology using low-cost materials as chromatography paper and polymeric sheets. Extra virgin olive oil (EVOO) phenolic compounds' represents a nutritional added value, nevertheless, the high lipidic content hinders their direct and rapid analysis, resulting in an extremely challenging sample. The realized lab-on-paper allows to perform the dual TPC and AOC determination in three simple steps: (i) sample loading, (ii) analytes transport to the analysis spot, and (iii) double colorimetric analysis exploiting the growth of AuNPs and AgNPs on paper mediated by phenolic compounds. Signal acquisition is achieved using a standard digital camera. The dual colorimetric assay is able to detect phenolic compounds in the 25-500 mg L^-1 range with limits of detection ≤6 mg L^-1 and good reproducibility (RSDs ≤11%). Direct analysis of EVOO samples (n = 30) correlated well (r > 0.92) with conventional spectrophotometric methods for TPC and AOC determination.

Gold-Modified Micellar Composites as Colorimetric Probes for the Determination of Low Molecular Weight Thiols in Biological Fluids Using Consumer Electronic Devices

This work describes a new, low-cost and simple-to-use method for the determination of free biothiols in biological fluids. The developed method utilizes the interaction of biothiols with gold ions, previously anchored on micellar assemblies through electrostatic interactions with the hydrophilic headgroup of cationic surfactant micelles. Specifically, the reaction of AuCl4− with the cationic surfactant cetyltrimethyl ammonium bromide (CTAB) produces an intense orange coloration, due to the ligand substitution reaction of the Br− for Cl− anions, followed by the coordination of the AuBr4− anions on the micelle surface through electrostatic interactions. When biothiols are added to the solution, they complex with the gold ions and disrupt the AuBr4−–CTAB complex, quenching the initial coloration and inducing a decrease in the light absorbance of the solution. Biothiols are assessed by monitoring their color quenching in an RGB color model, using a flatbed scanner operating in transmittance mode as an inexpensive microtiter plate photometer. The method was applied to determine the biothiol content in urine and blood plasma samples, with satisfactory recoveries (i.e., >67.3–123% using external calibration and 103.8–115% using standard addition calibration) and good reproducibility (RSD < 8.4%, n = 3).