Sensing colorimetric approaches based on gold and silver nanoparticles aggregation: Chemical creativity behind the assay. A review

Localized surface plasmon resonance (LSPR) is one of the most remarkable features of gold nanoparticles (Au NPs) and silver nanoparticles (Ag NPs). Due to these inherent optical properties, colloidal solutions of Au and Ag NPs have high extinction coefficients and different colour in the visible region of the spectrum when they are well-spaced in comparison with when they are aggregated. Therefore, a well-designed chemical interaction between the analyte and NPs surroundings leads to a change of colour (red to blue for Au NPs and yellow to brown for Ag NPs from well-spaced to aggregated ones, respectively) allowing the visual detection of the target analyte.     

Localized surface plasmon resonance sensing detection of glucose in the serum samples of diabetes sufferers based on the redox reaction of chlorauric acid

In this paper, the formation of gold nanoparticles (Au NPs) as a result of the thermo-active redox reaction of chlorauric acid (HAuCl₄) and glucose in alkaline medium was identified by measuring the plasmon resonance absorption, localized surface plasmon resonance (LSPR), and transmission electron microscopy (TEM) images, for the formation of Au NPs displays characteristic plasmon resonance absorption bands and corresponding LSPR signals. It was found that the resulted LSPR signals could be easily detected with a common spectrofluorometer. With increasing glucose concentration, the LSPR intensity displays linear response with the glucose content over the range from 2.0 to 250.0 μmol l⁻¹. Thus, a novel assay of glucose was established with the limits of determination (3σ) being 0.21 μmol l⁻¹, and the detection of glucose could be made easily in the serum samples of diabetes sufferers. Mechanism investigations showed that the activation energy and molar ratio of the reaction were 34.8 kJ mol⁻¹ and 3:2, respectively.     

SiO2/TiO2 Hollow Nanoparticles Decorated with Ag Nanoparticles: Enhanced Visible Light Absorption and Improved Light Scattering in Dye‐Sensitized Solar Cells

Hollow SiO₂/TiO₂ nanoparticles decorated with Ag nanoparticles (NPs) of controlled size (Ag@HNPs) were fabricated in order to enhance visible‐light absorption and improve light scattering in dye‐sensitized solar cells (DSSCs). They exhibited localized surface plasmon resonance (LSPR) and the LSPR effects were significantly influenced by the size of the Ag NPs. The absorption peak of the LSPR band dramatically increased with increasing Ag NP size. The LSPR of the large Ag NPs mainly increased the light absorption at short wavelengths, whereas the scattering from the SiO₂/TiO₂ HNPs improved the light absorption at long wavelengths. This enabled the working electrode to use the full solar spectrum. Furthermore, the SiO₂ layer thickness was adjusted to maximize the LSPR from the Ag NPs and avoid corrosion of the Ag NPs by the electrolyte. Importantly, the power conversion efficiency (PCE) increased from 7.1 % with purely TiO₂‐based DSSCs to 8.1 % with HNP‐based DSSCs, which is an approximately 12 % enhancement and can be attributed to greater light scattering. Furthermore, the PCEs of Ag@HNP‐based DSSCs were 11 % higher (8.1 vs. 9.0 %) than the bare‐HNP‐based DSSCs, which can be attributed to LSPR. Together, the PCE of Ag@HNP‐based DSSCs improved by a total of 27 %, from 7.1 to 9.0 %, due to these two effects. This comparative research will offer guidance in the design of multifunctional nanomaterials and the optimization of solar‐cell performance.     

Ultrasensitive optical detection of trinitrotoluene by ethylenediamine-capped gold nanoparticles

This study found that 1,2-ethylenediamine (EDA) as a primary amine could be modified onto the surface of citrate-stabilized gold nanoparticles (Au NPs), and the EDA-capped Au NPs were successfully used as an ultrasensitive optical probe for TNT detection. The strong donor–acceptor (D–A) interactions between EDA and trinitrotoluene (TNT) at the Au NP/solution interface induced significant aggregation of the EDA-capped Au NPs, and enabled to easily realize the direct colorimetric detection of ultratrace TNT. The results showed that such a color change was readily seen by the naked eye, and the colorimetric detection could be down to 400 pM level of TNT with excellent discrimination against other nitro compounds. UV–vis absorption spectroscopy was used to examine the TNT-induced changes in local surface plasmon resonance (LSPR) of EDA-capped Au NPs, and a new LSPR band at ca. 630 nm arose along with the addition of TNT, which produced a detection limit of TNT down to ca. 40 pM. Furthermore, dynamic light scattering measurements evidenced the ultratrace TNT-induced small changes in the size of the EDA-capped Au NPs, and realized the quick and accurate detection of TNT in 0.4 pM level. These results demonstrated the ultrahigh sensitivity of this optical probe for TNT detection. Moreover, this optical probe is sample, stable, low-cost, and these excellent properties make it quite promising for infield and rapid detection of TNT.     

Fine‐Tuning the Homometallic Interface of Au‐on‐Au Nanorods and Their Photothermal Therapy in the NIR‐II Window

The localized surface plasmon resonance (LSPR) of plasmonic nanomaterials is highly dependent on their structures. Going beyond simple shape and size, further structural diversification demands the growth of non‐wetting domains. Now, two new dimensions of synthetic controls in Au‐on‐Au homometallic nanohybrids are presented: the number of the Au islands and the emerging shapes. By controlling the interfacial energy and growth kinetics, a series of Au‐on‐AuNR hybrid structures are successfully obtained, with the newly grown Au domains being sphere and branched wire (nanocoral). The structural variety allowed the LSPR to be fine‐tuned in full spectrum range, making them excellent candidates for plasmonic applications. The nanocorals exhibit black‐body absorption and outstanding photothermal conversion capability in NIR‐II window. In vitro and in vivo experiments verified them as excellent photothermal therapy and photoacoustic imaging agents.     

Measurement of antibody binding to protein immobilized on gold nanoparticles by localized surface plasmon spectroscopy

Antibody binding to bovine serum albumin (BSA) and human serum albumin (HSA) immobilized onto gold nanoparticles was studied by means of localized surface plasmon resonance (LSPR) spectroscopy. Amine-modified glass was prepared by self-assembly of amine-terminated silane on substrate, and gold (Au) nanoparticles were deposited on the amine-modified glass substrate. Au nanoparticles deposited on the glass surface were functionalized by BSA and HSA. BSA immobilization was confirmed by LSPR spectroscopy in conjunction with surface-enhanced Raman scattering spectroscopy. Then, LSPR response attributable to the binding of anti-BSA and anti-HSA to BSA- and HSA-functionalized Au nanoparticles, respectively, was examined. Anti-HSA at levels larger than ∼10 nM could be detected by HSA-immobilized chips with LSPR optical response, which was saturated at concentrations greater than ∼650 nM of anti-HSA. Electronic supplementary material Supplementary material is available in the online version of this article at and is accessible to authorized users.     

Surface enhanced Raman scattering of p-aminothiophenol self-assembled monolayers in sandwich structure fabricated on glass

A sandwich structure consisting of Ag nanoparticles (NPs), p-aminothiophenol (p-ATP) self-assembled monolayers (SAMs), and Ag NPs was fabricated on glass and characterized by surface enhanced Raman scattering (SERS). The SERS spectrum of a p-ATP SAM in such sandwich structure shows that the electromagnetic enhancement is greater than that on Ag NPs assembled on glass. The obtained enhancement factors (EF) on solely one sandwich structure were as large as 6.0 +/- 0.62 x 10(4) and 1.2 +/- 0.62 x 10(7) for the 7a and 3b(b(2)) vibration modes, respectively. The large enhancement effect of p-ATP SAMs is likely a result of plasmon coupling between the two layers of Ag NP (localized surface plasmon) resonance, creating a large localized electromagnetic field at their interface, where p-ATP resides. Moreover, the fact that large EF values (approximately 1.9 +/- 0.7 x 10(4) and 9.4 +/- 0.7 x 10(6) for the 7a- and b(2)-type vibration modes, respectively) were also obtained on a single sandwich structure of Au NPsp-ATP SAMsAg NPs in the visible demonstrates that the electromagnetic coupling does not exist only between Ag NPs but also between Au and Ag NPs. The lower EF values on Au-to-Ag NPs compared to those on Ag-to-Ag NPs demonstrate that the Au-to-Ag coupling must be less effective than the Ag-to-Ag coupling for the induction of SERS in the visible.     

Multilayer gold nanoparticle-assisted protein tryptic digestion in solution and in gel under photothermal heating

Elevating the reaction temperature is an effective method to accelerate protein enzymatic digestion because it can promote protein denaturation and enzyme activities. In this study, we demonstrated a new photothermal heating method to assist protein tryptic digestion on glass slides. A glass slide coated with layer-by-layer gold nanoparticles (Glass@AuNPs), combined with the use of a near infrared (NIR) diode laser, was used to raise reaction temperature during tryptic digestion in a short period of time. The modified glass slide is capable of absorbing NIR light arising from the dipole–dipole interactions between Au NPs immobilized on the slide. The temperature of Glass@AuNPs rapidly increased when irradiated by the NIR laser, accelerating protein enzymatic digestion conducted on the slide. Thus, when performing the tryptic digestion of proteins on the Glass@AuNPs slide under NIR irradiation, 3.5 min was sufficient to carry out the tryptic digestion of proteins in solution, while less than 5 min was adequate for in-gel tryptic digestion of proteins. Matrix-assisted laser desorption/ionization mass spectrometry was used for characterization of the tryptic digestion product. On the basis of the results, the time taken to analyze proteins could be greatly reduced using this current approach.     

Iron oxide/tantalum oxide core–shell magnetic nanoparticle-based microwave-assisted extraction for phosphopeptide enrichment from complex samples for MALDI MS analysis

A new type of metal-oxide-coated magnetic nanoparticles (NPs)—tantalum-oxide-coated magnetic iron oxide (Fe₃O₄@Ta₂O₅) NPs—which are used as affinity probes for selectively trapping phosphopeptides from complex samples, is demonstrated in this study. In this approach, phosphopeptide enrichment was achieved by incubating the NPs with sample solutions under microwave heating within 1 min. The NP–target species conjugates were readily isolated from samples by magnetic separation followed by matrix-assisted laser desorption/ionization (MALDI) mass spectrometric analysis. When using human serum as the sample, phosphorylated fibrinopeptide-A-derived ions are the only ions observed in the MALDI mass spectra after enrichment by the Fe₃O₄@Ta₂O₅ NPs. Furthermore, only phosphopeptides appear in the MALDI mass spectra after using the affinity probes to selectively trap target species from the tryptic digest of a cell lysate and milk sample. The results demonstrated that the Fe₃O₄@Ta₂O₅ NPs have the capability of selectively trapping phosphorylated peptides from complex samples. The detection limit of this approach for a phosphopeptide (FQpSEEQQQTEDELQDK) was ~10 fmol. Figure For the first time, tantalum oxide-coated magnetic iron oxide (Fe₃O₄@Ta₂O₅) NPs were demonstrated as suitable affinity-probes for selectively trapping phosphopeptides from complex samples. To shorten the analysis time, phosphopeptide enrichment was achieved by incubating the NPs with sample solutions under microwave-heating within 1 min. MALDI MS was employed for characterization of the species trapped by the NPs.     

Au纳米粒子表面附近偶氮苯分子的吸收光谱

我们将时空间含时密度泛函理论与经典的电动力学时空间有限差分技术相结合,对吸附在Au纳米粒子表面附近的偶氮苯分子顺式和反式两种同分异构体的光学性质进行了研究。计算结果显示,一方面表面等离子激元共振会增大其周围激光外场的强度,从而使得偶氮苯分子的吸收光谱得到增强,另一方面,吸附分子与金属纳米粒子之间的相互耦合作用也会对一些低能特殊谱峰的强度及位置产生影响,从而使得谱线的形状与单独的偶氮苯分子或Au纳米粒子并不相同。此外,等离子激元的增强效应会随着分子与金属纳米粒子之间的距离以及纳米粒子尺寸的变化而发生变化。     

Interaction of plasmon and molecular resonances for rhodamine 6G adsorbed on silver nanoparticles

Localized surface plasmon resonance (LSPR) is a key optical property of metallic nanoparticles. The peak position of the LSPR for noble-metal nanoparticles is highly dependent upon the refractive index of the surrounding media and has therefore been used for chemical and biological sensing. In this work, we explore the influence of resonant adsorbates on the LSPR of bare Ag nanoparticles (lambda(max,bare)). Specifically, we study the effect of rhodamine 6G (R6G) adsorption on the nanoparticle plasmon resonance because of its importance in single-molecule surface-enhanced Raman spectroscopy (SMSERS). Understanding the coupling between the R6G molecular resonances and the nanoparticle plasmon resonances will provide further insights into the role of LSPR and molecular resonance in SMSERS. By tuning lambda(max,bare) through the visible wavelength region, the wavelength-dependent LSPR response of the Ag nanoparticles to R6G binding was monitored. Furthermore, the electronic transitions of R6G on Ag surface were studied by measuring the surface absorption spectrum of R6G on an Ag film. Surprisingly, three LSPR shift maxima are found, whereas the R6G absorption spectrum shows only two absorption features. Deconvolution of the R6G surface absorption spectra at different R6G concentrations indicates that R6G forms dimers on the metal surface. An electromagnetic model based on quasi-static (Gans) theory reveals that the LSPR shift features are associated with the absorption of R6G monomer and dimers. Electronic structure calculations of R6G under various conditions were performed to study the origin of the LSPR shift features. These calculations support the view that the R6G dimer formation is the most plausible cause for the complicated LSPR response. These findings show the extreme sensitivity of LSPR in elucidating the detailed electronic structure of a resonant adsorbate.     

Au nanorods decorated TiO2 nanobelts with enhanced full solar spectrum photocatalytic antibacterial activity and the sterilization file cabinet application

TiO₂ photocatalysts have been widely studied and applied for removing bacteria, but its antibacterial efficiency is limited to the ultraviolet (UV) range of the solar spectrum. In this work, we use the gold (Au) nanorods to enhance the visible and near-infrared (NIR) light absorption of TiO₂ NBs, a typical UV light photocatalyst, thus the enhancement of its full solar spectrum (UV, visible and NIR) photocatalytic antibacterial properties is achieved. Preliminary surface plasmon resonance (SPR) enhancement photocatalytic antibacterial mechanism is suggested. On one hand, transverse and longitudinal SPR of Au NRs is beneficial for visible and NIR light utilization. On the other hand, Au NRs combined with TiO₂ NBs to form the heterostructure, which can improve the photogenerated carrier separation and direct electron transfer increases the hot electron concentration while Au NRs as the electron channel can well restrain charge recombination, finally produces the high yield of radical oxygen species and exhibits a superior antibacterial efficiency. Furthermore, we design a sterilization file cabinet with Au NR/TiO₂ NB heterostructures as the photocatalytic coating plates. Our study reveals that Au NR/TiO₂ NB heterostructure is a potential candidate for sterilization of bacteria and archives protection.     

Two-step on-particle ionization/enrichment via a washing- and separation-free approach: multifunctional TiO2 nanoparticles as desalting, accelerating, and affinity probes for microwave-assisted tryptic digestion of phosphoproteins in ESI-MS and MALDI-MS: comparison with microscale TiO2

We introduce a simplified sample preparation method using bare TiO₂ nanoparticles (NPs) to serve as multifunctional nanoprobes (desalting, accelerating, and affinity probes) for effective enrichment of phosphopeptides from microwave-assisted tryptic digestion of phosphoproteins (α-casein, β-casein and milk) in Electrospray Ionization Mass Spectrometry (ESI-MS) and Matrix Assisted Laser Desorption Ionization Mass Spectrometry (MALDI-MS). The results demonstrate that TiO₂ NPs can effectively enrich and accelerate the digestion reactions of phosphoproteins in aqueous solutions and also from complex real samples. After the microwave experiments, we directly injected the resulting solutions into the ESI-MS and MALDI-MS systems for analysis, and excellent sensitivity was achieved without the need for any washing procedure or separation process. The reasons are attributed to the high binding affinity and selectivity of TiO₂ NPs toward phosphopeptides. Thus, phosphopeptides can be adsorbed onto the TiO₂ NP surface. The digested or partially digested phosphoproteins can be concentrated onto the TiO₂ NP surface. This results in the effective or complete digestion of phosphoproteins in a short period of time (45 s). In addition, high sensitivity and sequence coverage of phosphopeptide can be obtained using TiO₂ NPs as microwave absorbers and affinity probes in MALDI-MS and ESI-MS. This is due to the photocatalytic nature of the TiO₂ NPs because the absorption of microwave radiation that can accelerate the activation of trypsin for efficient digestion of phosphoproteins and enhances the ionization of phosphopeptides. The lowest concentrations detected for ESI-MS and MALDI-MS were 0.1 μM and 10 fmol, respectively, for α-casein. Comparing the two-step approach of TiO₂ NPs with microscale TiO₂ particles, the microscale TiO₂ particles shows no effect on the microwave-assisted tryptic digestion of phosphoproteins. The current approach offers multiple advantages, such as great simplicity, high sensitivity and selectivity, straightforward and separation/washing-free technique for phosphorpeptide enrichment analysis.     

Protoporphyrin IX‐Functionalized AgSiO2 Core–Shell Nanoparticles: Plasmonic Enhancement of Fluorescence and Singlet Oxygen Production

Metal‐enhanced processes arising from the coupling of a dye with metallic nanoparticles (NPs) have been widely reported. However, few studies have simultaneously investigated these mechanisms from the viewpoint of dye fluorescence and photoactivity. Herein, protoporphyrin IX (PpIX) is grafted onto the surface of silver core silica shell NPs in order to investigate the effect of silver (Ag) localized surface plasmon resonance (LSPR) on PpIX fluorescence and PpIX singlet oxygen (¹O₂) production. Using two Ag core sizes, we report a systematic study of these photophysical processes as a function of silica (SiO₂) spacer thickness, LSPR band position and excitation wavelength. The excitation of Ag NP LSPR, which overlaps the PpIX absorption band, leads to the concomitant enhancement of PpIX fluorescence and ¹O₂ production independently of the Ag core size, but in a more pronounced way for larger Ag cores. These enhancements result from the increase in the PpIX excitation rate through the LSPR excitation and decrease when the distance between PpIX and Ag NPs increases. A maximum fluorescence enhancement of up to 14‐fold, together with an increase in photogenerated ¹O₂ production of up to five times are obtained using 100 nm Ag cores coated with a 5 nm thick silica coating.     

Synthesis and Characterization of MWNTs/Au NPs/HS(CH2)6Fc Nanocomposite: Application to Electrochemical Determination of Ascorbic Acid

In this article, a detailed electrochemical study of a novel 6‐ferrocenylhexanethiol (HS(CH₂)₆Fc) self‐assembled multiwalled carbon nanotubes‐Au nanoparticles (MWNTs/Au NPs) composite film was demonstrated. MWNTs/Au NPs were prepared by one‐step in situ synthesis using linear polyethyleneimine (PEI) as bifunctionalizing agent. HS(CH₂)₆Fc, which acted as the redox mediator, was self‐assembled to MWNTs/Au NPs via Au‐S bond. Transmission electron microscopy (TEM), energy‐dispersive X‐ray analysis (EDX), Fourier transformed infrared absorption spectroscopy (FT‐IR), UV‐visible absorption spectroscopy, and cyclic voltammetry were used to characterize the properties of the MWNTs/Au NPs/HS(CH₂)₆Fc nanocomposite. The preparation of the nanocomposite was very simple and effectively prevented the leakage of the HS(CH₂)₆Fc mediator during measurements. The electrooxidation of AA could be catalyzed by Fc/Fc⁺ couple as a mediator and had a higher electrochemical response due to the unique performance of MWNTs/Au NPs. The nanocomposite modified electrode exhibited excellent catalytic efficiency, high sensitivity, good stability, fast response (within 3 s) and low detection limit toward the oxidation of AA at a lower potential.     

Organic–Inorganic Azafullerene‐Gold C59N‐Au Nanohybrid: Synthesis,Characterization, and Properties

Azafullerene (C₅₉N) was functionalized using a Mannich‐type reaction and then subsequently condensed with lipoic acid to yield dithiolane‐modified C₅₉N. In the following step, the extended dithiolane moiety from the C₅₉N core was utilized to decorate the azafullerene sphere with gold nanoparticles (Au NPs). The latter were initially stabilized with dodecanothiol (DT ? Au) and then integrated on azafullerene through a ligand exchange reaction with the dithiolane‐functionalized C₅₉N to produce the C₅₉N/DT ? Au nanohybrid. The nanohybrid was fully characterized by spectroscopy and microscopy, revealing the formation of spherical nanoparticles with a diameter in the range of 2–5 nm, as imaged by HR‐TEM. In the electronic absorption spectrum of C₅₉N/DT ? Au nanohybrid, the characteristic surface plasmon band (SPB) of Au NPs was observed, however, it was redshifted compared with that of DT ? Au. The redshift of the SPB is indicative of closer interparticle proximity of Au NPs, in accordance with the formation of aggregated NPs as observed by TEM, in C₅₉N/DT ? Au nanohybrid. Excited‐state interactions in C₅₉N/DT ? Au were probed by photoluminescence assays. It was found that the weak emission of C₅₉N at 819 nm was blueshifted by 14 nm in C₅₉N/DT ? Au, but was stronger in intensity, thus suggesting energy transfer to C₅₉N, within the organic–inorganic C₅₉N/DT ? Au nanohybrid. Finally, with the aid of pump–probe measurements and transient absorption spectroscopy, the formation of the singlet excited state of C₅₉N was identified.     

Plasmonic Titanium Nitride Tubes Decorated with Ru Nanoparticles as Photo-Thermal Catalyst for CO2 Methanation

Photo-thermal catalysis has recently emerged as a viable strategy to produce solar fuels or chemicals using sunlight. In particular, nanostructures featuring localized surface plasmon resonance (LSPR) hold great promise as photo-thermal catalysts given their ability to convert light into heat. In this regard, traditional plasmonic materials include gold (Au) or silver (Ag), but in the last years, transition metal nitrides have been proposed as a cost-efficient alternative. Herein, we demonstrate that titanium nitride (TiN) tubes derived from the nitridation of TiO₂ precursor display excellent light absorption properties thanks to their intense LSPR band in the visible–IR regions. Upon deposition of Ru nanoparticles (NPs), Ru-TiN tubes exhibit high activity towards the photo-thermal CO₂ reduction reaction, achieving remarkable methane (CH₄) production rates up to 1200 mmol g⁻¹ h⁻¹. Mechanistic studies suggest that the reaction pathway is dominated by thermal effects thanks to the effective light-to-heat conversion of Ru-TiN tubes. This work will serve as a basis for future research on new plasmonic structures for photo-thermal applications in catalysis.     

Water‐Soluble Gold Nanoparticles: From Catalytic Selective Nitroarene Reduction in Water to Refractive Index Sensing

Water‐soluble gold nanoparticles (Au NPs) stabilized by a nitrogen‐rich poly(ethylene glycol) (PEG)‐tagged substrate have been prepared by reduction of HAuCl₄ with NaBH₄ in water at room temperature. The morphology and size of the nanoparticles can be controlled by simply varying the gold/stabilizer ratio. The nanoparticles have been fully characterized by TEM, high‐resolution (HR) TEM, electron diffraction (ED), energy‐dispersive X‐ray spectroscopy (EDS), UV/Vis, powder XRD, and elemental analysis. The material is efficient as a recyclable catalyst for the selective reduction of nitroarenes with NaBH₄ to yield the corresponding anilines in water at room temperature. Furthermore, the potential ability of the Au NPs as a refractive index sensor owing to their localized surface plasmon resonance (LSPR) effect has also been assessed.     

Polypyrrole nanoparticles for high-performance in vivo near-infrared photothermal cancer therapy

Polypyrrole nanoparticles (PPy NPs) exhibit strong absorption in the near infrared (NIR) region. With an excellent photothermal efficiency of ～45% at 808 nm, sub-100 nm PPy NPs are demonstrated to be a promising photothermal agent for in vivo cancer therapy using NIR irradiation.     

Plasmon-Enhanced Electrochemiluminescence at the Single-Nanoparticle Level

Plasmon-enhanced electrochemiluminescence (ECL) at the single-nanoparticle (NP) level was investigated by ECL microscopy. The Au NPs were assembled into an ordered array, providing a high-throughput platform that can easily locate each NP in sequential characterizations. A strong dependence of ECL intensity on Au NP configurations was observed. We demonstrate for the first time that at the single-particle level, the ECL of Ru(bpy)₃²⁺-TPrA was majorly quenched by small Au NPs (<40 nm), while enhanced by large Au ones (>80 nm) due to the localized surface plasmon resonance (LSPR). Notably, the ECL intensity was further increased by the coupling effect of neighboring Au NPs. Finite Difference Time Domain (FDTD) simulations conformed well with the experimental results. This plasmon enhanced ECL microscopy for arrayed single NPs provides a reliable tool for screening electrocatalytic activity at a single particle.