A simple fluorescent bioassay for real time, ultrasensitive and highly selective determination of creatinine is extremely desirable in clinical science. Herein, we describe red-emittive bovine serum albumin-modified copper nanoclusters (BSA-CuNCs) applied as a precise fluorescent nanoprobe for the optical recognition of creatinine. The fluorescence of the BSA-CuNCs was considerably decreased by the addition of creatinine in a concentration-dependent manner. The observed experimental creatinine-induced emission quenching is attributed to the formation of non-fluorescent coordination complexes between creatinine and the BSA-CuNCs probe. The proposed fluorimetry approach enables the optical quantification of creatinine with a lowest detection limit of around 50.00 × 10⁻⁹ mol dm⁻³. In addition, the emission quenching ability of various potential interfering moieties for the BSA-CuNCs was investigated; only creatinine was found to be capable of successfully decreasing the emission spectral response of the BSA-CuNCs. Therefore, this creatinine-stimulated emission quenching can be utilized for the specific determination of creatinine using the BSA-CuNCs as a probe. The “turn-off” fluorescence characteristics and visible colour change to the naked eye of the probe under UV light illumination revealed the highly specific sensing of creatinine. Furthermore, this assay was successfully employed in the determination of creatinine in urine (human) samples, with very good recoveries (97.63% to 102.43%).

In this paper, a β-cyclodextrin (β-CD) functionalized gold nanoparticle (β-CD AuNP) based simple, rapid, on-site and naked eye colorimetric sensor for the sensitive detection of cysteine (Cys) was reported. The β-CD AuNP nanoprobe was completely characterized with respect to morphology, optical properties and stability. The surface plasmon resonance (SPR) and colorimetric properties of the β-CD AuNPs sensitively react to Cys, whereby the SPR band decreases with a red shift, resulting in a distinctive colour changes from wine red to purple. This colour change is easily visualized by the naked eye or absorption spectrophotometry. The observed spectral and colour changes are due to Cys having a thiol group, which has a high binding affinity towards the β-CD AuNP surface via Au–thiol bond formation, which leads to aggregation (SPR band and colorimetric changes). Further, the lowest limit of detection of Cys is calculated to be 25.47 × 10⁻⁹ mol dm⁻³ and this method exhibits a linear detection range of Cys from 2.50 to 45.00 × 10⁻⁷ mol dm⁻³. The β-CD AuNP based colorimetric sensor demonstrates high selectivity towards Cys over other essential amino acids. Moreover, the present naked eye visual method is further applied for the quantification of Cys in clinical samples, like human urine and blood serum. In addition, the present method was further extended to a smart phone device with easy changes of the device enabling it to act as a quantitative Cys detector.

Water toxicity, one of the major concerns for ecosystems and the health of humanity, is usually attributed to inorganic anions-induced contamination. Particularly, cyanide ions are considered one of the most harmful elements required to be monitored in water. The need for cyanide sensing and monitoring has tempted the development of sensing technologies without highly sophisticated instruments or highly skilled operations for the objective of in-situ monitoring. Recent decades have witnessed the growth of noble metal nanomaterials-based sensors for detecting cyanide ions quantitatively as nanoscience and nanotechnologies advance to allow nanoscale-inherent physicochemical properties to be exploited for sensing performance. Particularly, noble metal nanostructure e-based optical sensors have permitted cyanide ions of nanomolar levels, or even lower, to be detectable. This capability lends itself to analytical application in the quantitative detection of harmful elements in environmental water samples. This review covers the noble metal nanomaterials-based sensors for cyanide ions detection developed in a variety of approaches, such as those based on colorimetry, fluorescence, Rayleigh scattering (RS), and surface-enhanced Raman scattering (SERS). Additionally, major challenges associated with these nano-platforms are also addressed, while future perspectives are given with directions towards resolving these issues.

Herein, we describe a bright-red-emitting ovalbumin-protected gold nanoclusters (OVA–AuNCs) that were prepared and applied as a luminescent probe for a simple, rapid, and highly sensitive determination of cyanide ions (CN^– ions) based on an emission quenching and colorimetric method. Initially, an intense red-emissive fluorescence of the OVA–AuNCs successfully disappeared upon the addition of CN^– ions. The resultant emission-quenching process involved CN^– ions etching the OVA–AuNC surface, which produced AuCN₂^– complexes in the presence of ambient oxygen. Under optimized experimental conditions, the relative emission intensity is inversely relative to CN^– ion concentrations ranging from 5.00 × 10^–7 to 75.00 × 10^–7 mol/L with a linear correlation coefficient of 0.9932. Furthermore, OVA–AuNC-based optical detection systems on both colorimetric and fluorometric assays were tested, which expose highly sensitive and specific determination of CN^– ions, and it is easily visualized by the naked eye (day light and UV light). Because of the distinct Elsner reaction between Au atoms of OVA–AuNCs and CN^– ions, the recent nanoprobe offered ultrasensitivity and good selectivity with the lowest limit of detection value of 68.00 × 10^–9 mol/L. In addition, this fluorescence “turn-off” CN^– ion detection method was executed in real water samples. The demonstrated route of OVA–AuNC preparation is extremely easy and quick, making the proposed selective and sensitive CN^– ion sensing assay based on the fluorescence response of the OVA–AuNCs for numerous practical applications.

In this paper, we illustrate an efficient, convenient, and simplistic fluorescence technique for the specific identification for nitro explosive 2,4,6-trinitrophenol (TNP) in 100% water medium by bright cyan blue color-emitting poly(vinylpyrrolidone)-supported copper nanoclusters (PVP-CuNCs) as a fluorescence probe. PVP-CuNCs exhibited linear fluorescence quenching response toward the increasing concentration of TNP analyte. Surprisingly, TNP only reduces the emission signal of PVP-CuNCs, whereas various nitro explosives cause very slight reducing emission intensity, validating the good specificity of the PVP-CuNC probe toward TNP. The highest Stern–Volmer quenching constant (K_sv) value of 1.03 × 10⁷ dm³ mol^–1 and the extremely lowest limit of detection of 81.44 × 10^–12 mol dm^–3 were achieved solely for TNP in 100% water medium which is astonishing and exclusive for this nanoprobe. The sensing pathway for the high sensitivity of PVP-CuNCs assay to quantify the TNP is expected to combine with the inner filter effect process and static quenching. The static quenching mechanism between TNP and PVP-CuNCs is further verified by fluorescence decay measurements. Furthermore, the developed fluorescence sensing platform is applied for the quantification of a trace amount of TNP in real samples named dam water, sea water, and match stick.