Colorimetric recognition and sensing of thiocyanate with a gold nanoparticle probe and its application to the determination of thiocyanate in human urine samples

Nanozymatic detection of thiocyanate through accelerating the growth of ultra-small gold nanoparticles/graphene quantum dots hybrids

Thiocyanate (SCN^-) concentration monitoring in food is important to ensure the health and safety of the consumers.A colorimetric detection of thiocyanate (SCN^-) based on the nanozymatic activity of gold nanoparticle-graphene quantum dots (GQDs-Au NPs) hybrids in the presence of 3,3',5,5'-tetramethylbenzidine (TMB) and H₂O₂ has been proposed. Here, a new synthesis method of GQDs directly from graphite was introduced. Transmission electron microscopy (TEM) images revealed that the size of the GQDs was 3-5 nm, and the emission peak appeared at 450 nm. As-synthesized GQDs was utilized to produce GQDs-Au NPs hybrids without additional chemicals. However, the presence of SCN^- inhibits the growth of Au NPs, the resulting Au NPs are smaller in size. Moreover, SCN^- group is well-known for hydroxyl radical (OH) scavenging activity that could oxidize TMB. Both effects boosted the nanozymatic activity of GQDs-Au NPs to detect SCN^- under optimized conditions with a limit of detection (LOD) of 3 nM. Present study also validates the methodology to detect SCN^- in raw milk.

Positively Charged Gold Quantum Dots: An Nanozymatic "Off-On" Sensor for Thiocyanate Detection

The concentration of thiocyanate (SCN⁻) in bodily fluids is a good indicator of potential and severe health issues such as nasal bleeding, goiters, vertigo, unconsciousness, several inflammatory diseases, and cystic fibrosis. Herein, a visual SCN⁻ sensing method has been developed using the enzyme-like nature of positively charged gold quantum dots (Au QDs) mixed with 3,3′,5,5′-tetramethylbenzidine (TMB) and hydrogen peroxide (H₂O₂₎. This research also reports a new method of synthesizing positively charged Au QDs directly from gold nanoparticles through a hydrothermal process. Microscopic imaging has showed that the Au QDs were 3–5 nm in size, and the emission wavelength was at 438 nm. Au QDs did not display any enzyme-like nature while mixed up with TMB and H₂O₂. However, the nanozymatic activity of Au QDs appeared when SCN⁻ was included, leading to a very low detection limit (LOD) of 8 nM and 99–105% recovery in complex media. The steady-state kinetic reaction of Au QDs showed that Au QDs had a lower Michaelis–Menten constant (Km) toward H₂O₂ and TMB, which indicates that the Au QDs had a higher affinity for H₂O₂ and TMB than horseradish peroxidase (HRP). A mechanism study has revealed that the scavenging ability of hydroxyl (•OH) radicals by the SCN⁻ group plays an important role in enhancing the sensitivity in this study. The proposed nanozymatic “Off–On” SCN⁻ sensor was also successfully validated in commercial milk samples.

Optical detection of thiocyanate in human saliva based on the colorimetric response of (2-(2-hydroxyphenyl)-1H-benzo[d]imidazol-5-yl)(phenyl) methanone (HBPM)/Co2+ ions conjugate

Simple colorimetric biosensor was designed for the quantification SCN^- ions based on the principle of colorimetric reactions between Co²⁺ and SCN^- ions reaction using synthesized chromophore (2-(2-hydroxyphenyl)-1H-benzo[d]imidazol-5-yl)(phenyl) methanone (HBPM)/Co²⁺ ions conjugate which was synthesized via greener ultrawave sonication method. The structural characterization of the HBPM chromophore was confirmed by using NMR, ESI-MASS spectral techniques and the photophysical properties, sensor studies were done by using UV-Vis Spectrophotometer. Our designed HBPM/Co²⁺ ions conjugates have selectively detected SCN^- ions qualitatively and quantitatively in the presence of other human salivary interferents. As per clinical point of view, three different ranges of SCN^- ions from 0.1 to 2 mM for normal, 3 to 10 mM for disease, and 100-600 µM for below normal ranges were tested successfully by our developed sensor and the LoD was calculated as 5.43 nM. The real potent application of the developed biosensor was tested in human salivary samples of both smokers and non-smokers under different ages and obtained results shown good agreement with existing clinical methods.

How to identify a smoker: a salient crystallographic approach to detect thiocyanate content

There is an increasing demand for monitoring environmental pollutants and the control requires new sensing materials with better sensitivity, selectivity and reliability. In this study, a series of Co₇ clusters incorporating various flexible polyhydroxyamine ligands are explored, with the first report of thiocyanate recognition triggered by crystal formation using a Co₇ crystal (1). For this, we have fortunately synthesized three new mixed metal Co₇ clusters with fascinating structural features. The clusters were characterized by spectroscopic and single crystal X-ray diffraction methods and later by DFT calculations. Due to its better emission spectrum, 1 was further utilized for evaluating its sensing ability towards various anions in water. Surprisingly, 1 shows better quenching ability towards the recognition of SCN⁻ with a better binding constant. The luminescence quenching towards SCN⁻ detection was further verified by the single crystal method, HSAB principle (symbiosis) and theoretical calculations such as DFT studies. The SCXRD data clearly suggest that the Co₇ (1) can be converted into Co₁₄ (1a) by direct reaction with NaSCN under ambient conditions. Besides the soft/hard acid–base concept (symbiosis), the energies of formation, and Co–NCS and Co–OH₂ bond energies (as unravelled by DFT) are responsible for this transformation. Therefore, 1 can be used as a selective and sensitive sensor for the detection of thiocyanate anions based on the fluorescence amplification and quenching method. Further, the designed cluster has also been utilized to detect anions in human blood samples to differentiate a smoker and a non-smoker. It has been concluded that the samples of smokers have a high degree of thiocyanate (∼12 or 9.5 mg L⁻¹) in comparison to those of non-smokers (2–3 mg L⁻¹). Thus, this kind of cluster material has high potentiality in the field of bio-medical science in future endeavours for identification of the extent of thiocyanate content in smokers.

A new Co based sensor for thiocyanate recognition by formation of the dimeric crystals is designed to distinguish a smoker from a non-smoker.

Inhibited oxidase mimetic activity of palladium nanoplates by poisoning the active sites for thiocyanate detection

In this work, a novel convenient colorimetric method for sensitive detection of thiocyanate (SCN^-) has been developed based on its suppression of the oxidase-like activity of palladium square nanoplates on reduced graphene oxide (Pd SP@rGO). SCN^- can be adsorbed onto the surface of Pd SP@rGO via binding with Pd atoms and blocks the active sites that mimic oxidase, thus inhibiting the corresponding chromogenic reaction of 3,3',5,5'-tetramethylbenzidine, which has been comprehensively revealed by the UV-vis spectra and X-ray photoelectron spectra. The color fading exhibits SCN^- concentration-dependent behavior and can be easily recorded by either UV-vis spectroscopy or naked-eye observation. Therefore, both quantitative detection via measurement of the decrease in absorbance and visual detection of SCN^- can be achieved. Owing to the intrinsic amplification of signals by the oxidase-like activity of Pd SP@rGO without resorting to unstable and destructive H₂O₂, this assay is straightforward, robust and sensitive enough for the detection of SCN^- in real samples. Furthermore, an "INH" logic gate is rationally constructed based on the proposed colorimetric SCN^- sensor.

The small silver nanoparticle-assisted homogeneous sensing of thiocyanate ions with an ultra-wide window based on surface-enhanced Raman-extinction spectroscopy

For the first time, we present an original sensing strategy with an ultra-wide detection window from 17 nM to 20 mM to detect SCN- ions. Initially, we investigated the clustering and optical properties of noble metal sol nanoparticles (NPs) due to the competitive interaction of thiocyanate ions (SCN-) and cetyltrimethylammonium bromide (CTAB) under weak acidic conditions, and found that different dimensions and scales of nanoclusters containing the alkyne-embedded Au@Ag NPs and relatively small Ag NPs could be achieved by the mediation of CTAB through electrostatic forces and hydrophobic interaction, in which SCN- could be covalently bonded with the silver surface of NPs to form a compact molecular layer (-Ag-S-C[triple bond, length as m-dash]N), and CTAB could only occupy remaining sites. In this process, we found that SCN- always runs counter to CTAB and tends to dissolve nanoclusters, so that they occupy the exposed surface of NPs in nanoclusters rather than the binding sites of one another. Remarkably, when the concentration of SCN- initially increased, two highly recognizable SERS emissions, which were assigned to alkyne reporter molecules (2208 cm-1) and C[triple bond, length as m-dash]N of SCN- (2110 cm-1), respectively, were rapidly detected, and their ratios (I2110/I2208) increased linearly proportional to the concentration of SCN- over a range of 17 nM to 172 μM, with a limit of detection (LOD) of 10 nM. With the further increase of SCN-, small Ag NPs started to desorb from the surface of individual Au@Ag NPs and dissociated in the solution but did not contribute to SERS signals. Instead, the surface plasmon resonance (SPR) peak of pure silver NPs at 385 nm increased gradually in the range from 0.5 to 20 mM with an LOD of 0.2 mM. Of particular significance, this simple sensor in conjunction with surface-enhanced Raman-extinction spectroscopy can be used for the rapid detection of extensive samples with an ultra-wide detection window, such as body fluids (saliva, urine, and serum) and food (milk powder and brassica vegetables), which is far superior to that of ion chromatography (IC).

Detecting Mercury (II) and Thiocyanate Using "Turn-on" Fluorescence of Graphene Quantum Dots

In this work, 1.8 nm graphene quantum dots (GQDs), exhibiting bright blue fluorescence, were prepared using a bottom-up synthesis from citric acid. The fluorescence of the GQDs could be almost completely quenched (about 96%) by adding Hg²⁺. Quenching was far less efficient with other similar heavy metals, Tl⁺, Pb²⁺ and Bi³⁺. Fluorescence could be near quantitatively restored through the introduction of thiocyanate. This "turn-on" fluorescence can thus be used to detect both or either environmental and physiological contaminants mercury and thiocyanate and could prove useful for the development of simple point-of-care diagnostics in the future. Graphical Abstract.

Core-shell Au@Pt Nanoparticles Catalyzed Luminol Chemiluminescence for Sensitive Detection of Thiocyanate

In this study, core-shell Au@Pt nanoparticles (Au@Pt NPs) with peroxidase catalytic activity were synthesized by the seed-mediated method, and were used to catalyze the reaction of luminol-H₂O₂ to enhance the chemiluminescence (CL) intensity. It was found that thiocyanate (SCN^-) can effectively inhibit the catalytic activity of Au@Pt NPs. Based on this phenomenon, a method to detect SCN^- by using the Au@Pt NPs-catalytic luminol-H₂O₂ CL system was established, which has an ultra-low detection limit and an ultra-wide linear range, as well as the advantages of being simple and having low-cost and convenient operation. The research mechanism indicated that SCN^- could be adsorbed on the surface of Au@Pt NPs and occupies the active sites of Pt nanostructures, which led to a decrease in the amount of Pt⁰ and a loss of the excellent catalytic activity of Au@Pt NPs. After optimizing the experimental conditions, this assay for detecting SCN^- exhibited a good linear range from 5 to 180 nM, and the low detection limit was 2.9 nM. In addition, this approach has been successfully applied to the detection of SCN^- in tap-water samples, which has practical application value and embodies good development prospects.

Comparative study of serum sample preparation methods in aggregation-based plasmonic sensing

The use of nanoparticle-based colorimetric methods has received considerable attention for clinical analysis in biofluids. Sample preparation methods are surveyed here to minimize the impact of the sample matrix on the performance of a nanoparticle aggregation assay.

Lewis basicity of relevant monoanions in a non-protogenic organic solvent using a zinc(ii) Schiff-base complex as a reference Lewis acid

The Lewis basicity of relevant anions is reported for the first time and compared with that of neutral bases.

A dual-mode colorimetric and fluorometric “light on” sensor for thiocyanate based on fluorescent carbon dots and unmodified gold nanoparticles

A novel, highly sensitive and selective dual-readout (colorimetric and fluorometric) sensor based on fluorescent carbon dots (CDs) and unmodified gold nanoparticles (AuNPs) for the detection of thiocyanate (SCN⁻) was proposed.

Photochemically mediated synthesis of a gold colloid by dithizone and its application in the amperometric sensing of thiocyanate

Au–DTZH was synthesized by a one-step photochemical route and used for the amperometric sensing of thiocyanate. The modified electrode has a sensitivity of 16 nA nM⁻¹ and a limit of detection of 23.35 nM at a potential of 0.55 V vs. Ag/AgCl.

Ultrasensitive turn-on fluorescent detection of trace thiocyanate based on fluorescence resonance energy transfer

Thiocyanate (SCN(-)) is a small anion byproduct of cyanide metabolism. Several methods have been reported to measure SCN(-) above the micromolar level. However, SCN(-) is derived from many sources such as cigarettes, waste water, food and even car exhaust and its effect is cumulative, which makes it necessary to develop methods for the detection of trace SCN(-). In this paper, a simple and ultrasensitive turn-on fluorescence assay of trace SCN(-) is established based on the fluorescence resonance energy transfer (FRET) between gold nanoparticles (AuNPs) and fluorescein. The detection limit is 0.09 nM, to the best of our knowledge, which has been the lowest detection LOD ever without the aid of costly instrumentation. The fluorescence of fluorescein is significantly quenched when it is attached to the surface of AuNPs. Upon the addition of SCN(-), the fluorescence is turned on due to the competition action between SCN(-) and fluorescein towards the surface of AuNPs. Under an optimum pH, AuNPs size and concentration, incubation time, the fluorescence enhancement efficiency [(IF-I0)/I0] displays a linear relationship with the concentration of SCN(-) in the range of 1.0 nM to 40.0 nM. The fluorescein-AuNP sensor shows absolutely high selectivity toward SCN(-) than other 16 anions. The common metal ions, amino acids and sugars have no obvious interference effects. The accuracy and precision were evaluated based on the recovery experiments. The cost effective sensing system is successfully applied for the determination of SCN(-) in milk products and saliva samples.

A “light-on” colorimetric assay for anion detection using the inhibitory effect of anions on the catalytic activity of metal nanoparticles for the degradation of methyl orange

A “light-on” colorimetric method for anion detection using the catalytic ability of metal nanoparticles for methyl orange (MO) degradation was developed.