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We propose a convenient and easy colorimetric assay for highly sensitive detection of iodide by using gold nanostars (GNSs) as probes. The assay relies on that iodide directly changes the morphology of GNSs and alters their longitudinal localized surface plasmon resonance (LSPR) without surface modifications and the use of other reagents. Upon increasing iodide concentration, GNSs gradually transformed to sphere gold nanoparticles, the absorbance at longitudinal LSPR decreased, and solution color varied from greenish blue to red, as confirmed by the UV-Vis absorption spectroscopy and transmission electron microscopy. With this strategy, as low as 0.005 μM of iodide can be determined due to the specific properties of GNS with plenty of tips and corners and high surface-to-volume ratio. The detection was simply achieved by mixing testing samples and GNS solution. Many ions like CO32-, S2- did not interfere with iodide detection since only iodide can trigger GNS geometry change through an electron injection process. The iodide contents in river water, table salt, seaweed, and complex vitamin tablet were quantified with great accuracy. The proposed assay shows great promises for environment protection and food safety. Moreover, GNSs are useful in developing colorimetric assays for biochemical analysis or clinical diagnosis.

Gold nanorods (GNRs) with desired longitudinal localized surface plasmon resonance (LLSPR) and strong scattering intensity are important for extending their practical applications in bioimaging and sensing. Herein, a simple additive (HCl and Na2S)-regulated overgrowth approach has been proposed for preparing GNRs with tunable LLSPR. In this approach, HCl is used to slow down the growth reaction rate by changing chemical equilibrium, while Na2S is utilized to halt the reaction when LLSPR is reaching the expected wavelength under monitoring by a UV-Vis spectrometer. Under optimal conditions, GNRs with an LLSPR range from 850 to 650 nm could be facilely prepared with a high precision of 3 nm deviation. The TEM images reveal that GNRs have high monodispersity, displaying an increase in both length and diameter but a decrease in the aspect ratio. With the increase in size, the produced GNRs show enhanced scattering intensity and are applicable for single nanoparticle imaging due to the enlarged absorption and scattering cross-section and improved matching efficiency toward the CCD response.

The sensitivity for analytes of interest is vital for environment protection and food safety. Here, we propose an extremely sensitive assay toward Pb2+ by using gold nanostars (GNSs) as probes based on the catalytic activity of Pb on etching gold atoms after being reduced in the presence of 2-mercaptoethanol (2-ME) and sodium thiosulfate. GNSs were prepared by using 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid as both the reducing and capping agents, enabling high stability and sensitivity for quantitation of Pb2+. Upon increasing Pb2+ concentration over the range of 0–10 μM, GNS solution color changed from greenish-blue to blue to purple to red, and eventually to colorless. The color change can be distinguished by naked eye at the Pb2+ concentration as low as 200 pM. Through monitoring longitudinal localized surface plasmon of GNSs, Pb2+ could be detected with a limit of detection of 1.5 pM, and the working range is 2 pM-1 μM. The ultra-high sensitivity of our assay stems from the high catalysis of Pb on etching gold on tips and branches in the presence of 2-ME and sodium thiosulfate, leading to the shape deformation to spherical gold nanoparticle and the corresponding significant changes in their optical properties. The assay provides high selectivity of Pb2+ over the tested interfering metal ions like Cu2+. With high sensitivity and selectivity, the assay was efficiently validated by analyzing water samples and monitoring the migration of Pb2+ from the tested container to water.

Gold nanorods (GNRs) with desired longitudinal localized surface plasmon resonance (LLSPR) and strong scattering intensity are important for extending their practical applications in bioimaging and sensing. Herein, a simple additive (HCl and Na

The development of new methods for applications of detection in the biological field is a topic of intense and growing interest. The currently practiced methods for biological detection have many defects, such as time-consuming, low accuracy, and cumbersome for operators. Gold nanorods (AuNRs) have received tremendous attention recently in the fields of biological detection owing to the unique characteristics of surface plasmon resonance (SPR), which provides a remarkable ability to absorb and scatter light. Furthermore, AuNRs can be functionalized with a wide variety of biomolecules for targeted detection. Moreover, their versatility and unique properties have generated more applications in medical areas. In this review, we briefly outline the synthesis and surface modification of AuNRs, and focus on the recent advances of the application of AuNRs in detection field. Finally, the outlook and future prospect on the development of AuNRs are provided.

Preparation of carbon dots (C dots) with strong emission at long wavelengths is an ongoing goal to their advanced applications, especially in bioanalysis. For sensing Fe3+ and ascorbic acid (AA), we prepared C dots with a large Stokes shift using o-phenylenediamine (OPD) as the carbon source through an electrochemical method. During the electrolysis, OPD undergoes polymerization, carbonization, and passivation at the platinum anode to form C dots. These C dots were characterized by UV–Vis spectroscopy, transmission electron microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and FT-IR spectroscopy. They are composed of C, N, O, H elements, having a maximum of emission at 570 nm and a supreme excitation at 261 nm, with a Stokes shift of 309 nm. A new emission at 350 nm emerged with their exposure to Fe3+ and the intensity rises upon increasing Fe3+, while the emission intensity at 570 nm decreases. Thus, dissimilar to the usual quenching strategy, a novel ratiometric fluorescent sensor based on the fluorescence intensity ratio (F350/F570) of the C dots at 350 nm and 570 nm has been developed for quantitation of Fe3+ with a limit of detection (LOD) of 0.16 μM and high selectivity toward Fe3+ against K+, Cu2+, and Fe2+. With evidence of decreased C–N C and C–O/N–O in XPS and increased content of C O/N O and N–H, we suggested the complexation of Fe3+ with the surface groups led to the changes in chemical bonds accounting for their optical property variation. Furthermore, AA can quench the fluorescence of C dots at 570 nm and Fe3+-treated C dots (incubated with 50 μM Fe3+) at 350 nm by a static mechanism. Accordingly, two assays toward AA were developed with LODs of 0.12 μM and 0.32 μM by using C dots and Fe3+-treated C dots, respectively. Especially, both AA assays exhibited excellent selectivity under various substances including those with strong reducing ability such as pyrogallic acid, sodium citrate, tea polyphenol, and dithiothreitol. The applications of the assays were validated with the quantitation of Fe3+ in urine and AA in beverages. Electrochemical method is a significant approach to develop novel C dots even using the same carbon sources as in other methods.

Gold nanomaterial (GNM)-based colorimetric sensors have gained particular attention in the analytical field because of their instrument-free, low-cost, and easy-to-use features. Herein, a universal strategy for improving the sensitivity of colorimetric assays, which is based on GNM etching, was proposed by using additives to shift the chemical equilibrium of the reactions. As a proof-of-concept demonstration, nitrite was detected with high sensitivity using gold nanorod (AuNR) by monitoring the shift of its longitudinal localized surface plasmonic resonance (Δλ) with the use of NH4Br and HCl. These two additives could change chemical equilibriums and promote the etching of more gold atoms at the AuNR ends. The assay is sensitive and 0.001 μM of nitrite could result in Δλ change by 3 nm. A linear relationship between Δλ and nitrite concentration was established (Δλ =22.8∗Cnitrite +9.7) in the range of 0.016–8.0 μM with a coefficient of 0.999. The reaction time was also accurately controlled by adjusting solution pH with NH3⋅H2O to terminate the test. The sensitivity of this colorimetric assay toward nitrite was significantly higher than that of the conventional Griess assay, while it was the highest among the reported GNM-based approaches. This method was successfully applied in determining the nitrite content in food samples, including waters, spinach, and rose petals, with less rigorous sample processing. The proposed strategy of using additives to alter chemical equilibriums to improve sensitivity could be extended to other GNM etching sensors.