A study of reaction between quinones and thiourea: determination of thiourea in orange juice

A novel and simple naphthol azo dye chemosensor as a naked eye detection tool for highly selective, sensitive and accurate determination of thiourea in tap water, juices and fruit skins

In the present study, a highly accurate and sensitive azo-dye-based colorimetric sensor based on Eriochrome Black T (EBT) was proposed to detect and determine thiourea (TU). TU is truly an important toxic and carcinogenic hazardous pollutant as approved by EPA and IARC. This chemosensor shows a distinct color change from blue to pink during interaction with TU in aqueous medium. So EBT is capable as an applied tool for naked eye detection of TU as its color change is easily observed without any means. The sensing mechanism was also investigated using UV-vis absorption and FT-IR spectra. The linear range and the detection limit of TU sensing were respectively 0.15-18.5 μmol/L and 0.02 μmol/L. In addition, the relative standard deviation (RSD) based on ten repetitions calculated for two different TU concentrations 4.4 and 9.0 μmol/L were 2.3 % and 1.8 %, respectively. Besides its useful application as a naked eye detection tool, the advantages of the developed method include simplicity, elimination of tedious separation and pre-concentration steps, executable in neutral aqueous media, low costs, high accuracy, linear response for wide range of concentrations, low detection limit, high sensitivity, compatibility, and excellent selectivity. The concentration of TU in tap water, fruit juices or fruit skin samples can be visually detected and determined easily using this method. The results showed that EBT is an ideal colorimetric chemosensor for TU, which has been reported for the first time.

A fluorescent probe based on FRET effect between carbon nanodots and gold nanoparticles for sensitive detection of thiourea

Illegal abuse results in the presence of thiourea (TU) in soil, wastewater, and even fruits, which is harmful for the environment and human health. It has urgent practical significance to design an efficient and reliable probe for TU detection. Herein, a sensitive fluorescent probe with off-on response for harmful TU was reported. The probe was designed with fluorescent carbon nanodots (CNDs) and gold nanoparticles (AuNPs) based on fluorescence resonance energy transfer (FRET) effect. Firstly, the CNDs were pre-combined with AuNPs and the fluorescence of CNDs was quenched due to the FRET effect. Upon addition of TU, the fluorescence of CNDs recovered due to the unbinding of CNDs and AuNPs, since the coordination interaction between TU and AuNPs is stronger than the electrostatic interaction among CNDs and AuNPs. Under the optimum parameters, a linear relationship was found between the relative fluorescence intensity of the probe and the concentration of TU in the range of 5.00 × 10^-8-1.00 × 10^-6 M (R² = 0.9958), with the limit of detection (LOD) calculated to be 3.62 × 10^-8 M. This proposed method is easy to operate and has excellent selectivity and sensitivity for TU, which can be effectively applied in environmental water and compound fruit-vegetable juice.

Sensitive Detection of Thiourea Hazardous Toxin with Sandwich-Type Nafion/CuO/ZnO Nanospikes/Glassy Carbon Composite Electrodes

In this research study, we developed a voltammetric electrochemical sensor probe with a copolymer Nafion (Sulfonated Tetrafluoroethylene-based Fluoro-polymer) decorated with hydrothermally prepared sandwich-type CuO/ZnO nanospikes (NSs) onto a glassy carbon electrode (GCE) for reliable thiourea (TU) detection. The detailed characterizations in terms of structural morphology, binding energy, elemental compositions, grain size and crystallinity for synthesized NSs were performed by field emission scanning electron microscopy (FESEM), X-ray photoelectron spectroscopy (XPS), energy-dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD) analysis, respectively. The differential pulse voltammetric (DPV) analysis for TU showed good linearity at current-versus-TU concentration on the calibration plot in the 0.15~1.20 mM range, which is defined as a dynamic detection range (LDR) of TU in a phosphate buffer solution. Considering the slope of LDR over the GCE-coated NSs surface area (0.0316 cm2), the TU sensor sensitivity (0.4122 µA µM-1 cm-2) was obtained. Besides this, the low limit (LOD) for TU detection was calculated and found to be 23.03 ± 1.15 µM. The fabricated Nafion/CuO/ZnO NSs/GCE sensor probe was created as a reliable sensor based on reproducibility, interference effect, stability and response time. Real bio-samples were investigated and the results confirm the anticipated reliability of the TU sensor probe. Thus, this is a noble way to develop enzyme-free electrochemical sensors that could be an alternative approach for the detection of chemicals in the field of enzyme-free biosensor development technology.

'Urease immobilized single-kit' for sensing of thiourea-glucose pair employing fluorescence 'Turn off - Turn on' and as an efficient sorbent for selective sample cleanup of thiourea

The tenfold lowering in binding energy for TU-Tyrosine in immobilized urease (K_b: 4.7 × 10³) with respect to the native enzyme (K_b: 6.5 × 10⁴) begets easy desorption of thiourea (TU) by glucose (GL) with an eventual formation of a more strong TU- GL adduct; that rejuvenates the kit-material ready for the subsequent cycle(s). The sorption-desorption heeds fluorescence turn-off and turn-on in DCM for selective sensing of TU- GL pair at their respective linear range of concentration 2.5-26.1 ppm and 2.36-11.57 ppm. The process was found to be static (K_SV ≥ 2.25 × 10³ L mol^-1), exothermic (ΔH: -0.08 kJ mol^-1), spontaneous (ΔG: -21.1 kJ mol^-1) and marginally entropy gaining (ΔS: 0.07 kJ mol^-1 K^-1). The 'bulk material' (200 ± 20 μm) brilliantly preconcentrates TU with an enrichment factor of 106.2 after its selective extraction at near-neutral pH from a large volume sample (800 mL) of low concentration (30 ppm). A very dilute solution (0.05 mmol L^-1) of GL at minimum volume (6 mL) acts as a stripping agent and provides a longer life (200 cycles with good extraction efficiency) to the material. The method was found to be efficient in the analysis of fruit juice as a real sample.

Size-modulated optical property of gold nanorods for sensitive and colorimetric detection of thiourea in fruit juice

As an important sulfur compound, thiourea (TU) has caused great concern because of its wide application as well as its serious toxicity and hazard to the environment. Thus, it is necessary to develop a sensitive and selective method for TU analysis. In this work, gold nanorods (AuNRs) acted as an optical probe to realize the sensitive and colorimetric detection of TU. In HCl medium, Fe³⁺ at low concentration was difficult to oxide Au⁰ to form Au⁺ because of the high redox potential or the positive Gibbs free energy change. However, this process was possible when TU was present since the association constant between Au⁺ and TU is great enough to bind with TU to form a stable complex to further promote the etching of AuNRs, resulting in the lower aspect ratio of AuNRs with the blue shift and intensity decrease in extinction spectra, accompanied by the divisive colors of AuNRs solution or colorful dark-field light scattering imaging of single AuNR. The blue-shift of AuNRs longitudinal plasmon resonance absorption (LPRA) band was proportional to the concentration of TU in the range of 1-250 nM and the limit of detection (3σ/k) was as low as 0.4 nM. In addition, the colorimetric method was proven with high selectivity in the presence of potential interfering compounds, which was successfully applied to the detection of TU in fruit juice samples. This proposed colorimetric method provides a simple, sensitive yet selective measurement tool for TU sensing, which may offer new opportunities in the development of colorimetric sensors for food safety in the future.

Identifying three routes of the sensing mechanism for casein-directed gold nanoclusters

Although there have existed kinds of gold nanoclusters (AuNCs) employed for various sensing purposes, scarce studies were proposed for systematically exploring their multiple routes of the sensing mechanism since that plays the critical role for broadening their analysis applications. Herein, casein-directed gold nanoclusters (AuNCs@casein) have been successfully raised for identifying their multiple routes towards the sensing mechanism. Initially, the current nanoclusters were synthesized in aqueous solution with a quantum yield of nearly 2.1%, which obviously showed the red fluorescence at 600 nm. Subsequently, we have explored three routes of quenching the fluorescence of AuNCs@casein by introducing thiourea, bromelain, and graphene oxide with amino groups (GO-NH₂) as the representatives. To be specific, the formed bonds of Au-S between thiourea and AuNCs@casein achieved to quench their fluorescence, and the templates of AuNCs@casein destroyed by bromelain also showed the dramatically decreased fluorescence, and the energy transferring from the current AuNCs to GO-NH₂ indeed lead to the fluorescent variations of AuNCs@casein. On the basis of these phenomena, AuNCs@casein was further applied for simply sensing thiourea, bromelain and GO-NH₂ through the standard formats. Significantly, this study elaborated three quenching routes of AuNCs@casein and may provide more sensing strategies for other types of nanoclusters.

A dual-mode signaling response of a AuNP-fluorescein based probe for specific detection of thiourea

By employing fluorescein and AuNPs as energy donors and acceptors, respectively, a novel fluorescence resonance energy transfer (FRET)-based dual-mode sensor for selective recognition and quantitative detection of thiourea was designed and constructed in this study for the first time.

A novel fluorescence probe based on p-acid-Br and its application in thiourea detection

A novel organic fluorescence system was developed to detect thiourea based on p-acid-Br, which in the range of 0.5–1000 nM and with a detection limit of 0.26 nM. And this method provides a new promising platform for clinical analysis.

An on-line spectrophotometric determination of trace amounts of thiourea in tap water, orange juice, and orange peel samples using multi-channel flow injection analysis

In this work, a flow injection analysis (FIA) method was introduced for the determination of trace amounts of thiourea in tap water. This method is based upon the inhibition effect of thiourea on the reaction between meta-cresol purple (MCP) and potassium bromate catalyzed by bromide ions in a sulfuric acid medium. In the presence of thiourea, an induction period appears in the reaction system, and as a result, the absorbance of MCP increases at 525 nm in the FIA manifold. The chemical and FIA variables are studied and optimized using the univariate and Simplex optimization methods. Under the optimum conditions, thiourea can be determined in the range of 0.100-13.0 μg mL(-1). The limit of detection (3σ) for thiourea was found to be 0.0310 μg mL(-1). The relative standard deviations (RSDs) for six replicate determinations of 0.500, 5.00, and 12.0 μg mL(-1) of thiourea were 4.0%, 1.8%, and 1.2%, respectively. The proposed method was also applied for the determination of thiourea in orange juice and orange peel samples with recoveries in the range of 98.0-101%. The analytical speed of the method was calculated to be about 120 sample per hour.

Affinity covalent immobilization of glucoamylase onto ρ-benzoquinone activated alginate beads: I. Beads preparation and characterization

ρ-Benzoquinone-activated alginate beads were presented as a new carrier for affinity covalent immobilization of glucoamylase enzyme. Evidences of alginate modification were extracted from FT-IR and thermal gravimetric analysis and supported by morphological changes recognized through SEM examination. Factors affecting the modification process such as ρ-benzoquinone (PBQ) concentration, reaction time, reaction temperature, reaction pH and finally alginate concentration, have been studied. Its influence on the amount of coupled PBQ was consequently correlated to the changes of the catalytic activity and the retained activity of immobilized enzyme, the main parameters judging the success of the immobilization process. The immobilized glucoamylase was found kept almost 80% of its native activity giving proof of non-significant substrate, starch, diffusion limitation. The proposed affinity covalent immobilizing technique would rank among the potential strategies for efficient immobilization of glucoamylase enzyme.

Determination of thiourea in fruit juice by a kinetic spectrophotometric method

A catalytic kinetic method is described for determination of trace levels of thiourea based on its catalytic effect on the oxidation of Janus green (JG) by potassium iodate in hydrochloric acid media. The reaction was monitored by measuring the decrease in absorbance of the dye at 610 nm after 25 min. The effect of some factors on the reaction speed was investigated. The developed method allowed the determination of thiourea in range of 0.01-12.00 mg L(-1) with good precision, accuracy and the detection limit was 0.008 mg L(-1). Most of foreign species do not interfere with the determination. The method was found to be sensitive, selective and was applied to the determination of thiourea in fruit juices and orange peel.

A kinetic method for the determination of thiourea by its catalytic effect in micellar media

A highly sensitive, selective and simple kinetic method was developed for the determination of trace levels of thiourea based on its catalytic effect on the oxidation of janus green in phosphoric acid media and presence of Triton X-100 surfactant without any separation and pre-concentration steps. The reaction was monitored spectrophotometrically by tracing the formation of the green-colored oxidized product of janus green at 617 nm within 15 min of mixing the reagents. The effect of some factors on the reaction speed was investigated. Following the recommended procedure, thiourea could be determined with linear calibration graph in 0.03-10.00 microg/ml range. The detection limit of the proposed method is 0.02 microg/ml. Most of foreign species do not interfere with the determination. The high sensitivity and selectivity of the proposed method allowed its successful application to fruit juice and industrial waste water.

Selective thiourea optical probe based on thiourea-induced removal of chloroacetyl group from chloroacetylamine

A novel chromogenic probe for thiourea, N-chloroacetyl parafuchsin (CAP), was designed and synthesized. The method was based on the reaction of CAP with thiourea, resulting in the initial formation of the S-substituted thioformamidine hydrochloride, which would then undergo the intramolecular amidinolysis to liberate the corresponding amine, and thus leading to an absorbance increase at visible spectral range. Based on this mechanism, a highly selective optical probe for thiourea was developed. Under optimal conditions, absorbance increase (DeltaA) at 562 nm is proportional to thiourea concentration up to 0.2 mmolL(-1) with a detection limit of 2.1 micromolL(-1) (3sigma). Because of the selective removal of chloroacetyl group from CAP by thiourea, there is little interference by other molecules.