A petavoxel fragment of human cerebral cortex reconstructed at nanoscale resolution.

To fully understand how the human brain works, knowledge of its structure at high resolution is needed. Presented here is a computationally intensive reconstruction of the ultrastructure of a cubic millimeter of human temporal cortex that was surgically removed to gain access to an underlying epileptic focus. It contains about 57,000 cells, about 230 millimeters of blood vessels, and about 150 million synapses and comprises 1.4 petabytes. Our analysis showed that glia outnumber neurons 2:1, oligodendrocytes were the most common cell, deep layer excitatory neurons could be classified on the basis of dendritic orientation, and among thousands of weak connections to each neuron, there exist rare powerful axonal inputs of up to 50 synapses. Further studies using this resource may bring valuable insights into the mysteries of the human brain.

Boosting the Electroreduction of CO2 to CO by Ligand Engineering of Gold Nanoclusters.

The electrochemical CO2 reduction reaction (CO2RR) has been widely studied as a promising means to convert anthropogenic CO2 into valuable chemicals and fuels. In this process, the alkali metal ions present in the electrolyte are known to significantly influence the CO2RR activity and selectivity. In this study, we report a strategy for preparing efficient electrocatalysts by introducing a cation-relaying ligand, namely 6-mercaptohexanoic acid (MHA), into atom-precise Au25 nanoclusters (NCs). The CO2RR activity of the synthesized Au25(MHA)18 NCs was compared with that of Au25(HT)18 NCs (HT = 1-hexanethiolate). While both NCs selectively produced CO over H2, the CO2-to-CO conversion activity of the Au25(MHA)18 NCs was significantly higher than that of the Au25(HT)18 NCs when the catholyte pH was higher than the pKa of MHA, demonstrating the cation-relaying effect of the anionic terminal group. Mechanistic investigations into the CO2RR occurring on the Au25 NCs in the presence of different catholyte cations and concentrations revealed that the CO2-to-CO conversion activities of these Au25 NCs increased in the order Li+ < Na+ < K+ < Cs+, and are gated by the cation-coupled electron transfer step. These results were confirmed by the Nernstian shifts of the polarization curves at different cation concentrations.

Synthesis of RhH-doped Au-Ag alloy nanoclusters and dopant evolution.

Doping atomically precise metal nanoclusters (NCs) with heterometals is a powerful method for tuning the physicochemical properties of the original NCs at the atomic level. While the heterometals incorporated into metal NCs are limited to group 10-12 metals with closed d-shells, the doping of open d-shell metals remains largely unexplored. Herein, we report the synthesis of Rh-doped Au-Ag alloy NCs by a metal-exchange reaction of [RhHAg24(SPhMe2)18]2- NCs with an Au-thiolate complex. Combined experimental and theoretical structural studies revealed that the synthesized product is a dianionic [RhHAuxAg24-x(SPhMe2)18]2- NC (x = 8-12), consisting of RhH dopant, Au-rich kernel, and Ag-thiolate staple motifs, with the superatomic 8-electron configuration (1S21P6). Under aerobic conditions, the synthesized NCs underwent kernel evolution to generate a 6-electron [RhAuxAg24-x(SPhMe2)18]1- NC (1S21P4), which was initiated by the desorption of hydride from the kernel. Structural analysis of the [RhHAuxAg24-x(SPhMe2)18]2- NC suggests that the kernel evolution is induced by the change in chemical bonds surrounding the hydride in the Au-rich kernel.

ClAg14 (C≡Ct Bu)12 Nanoclusters as Efficient and Selective Electrocatalysts Toward Industrially Relevant CO2 Conversion.

Atomically precise metal nanoclusters (NCs) have emerged as a promising frontier in the field of electrochemical CO2 reduction reactions (CO2 RR) because of their distinctive catalytic properties. Although numerous metal NCs are developed for CO2 RR, their use in practical applications has suffered from their low-yield synthesis and insufficient catalytic activity. In this study, the large-scale synthesis and electrocatalytic performance of ClAg14 (C≡Ct Bu)12 + NCs, which exhibit remarkable efficiency in catalyzing CO2 -to-CO electroreduction with a CO selectivity of over 99% are reported. The underlying mechanisms behind this extraordinary CO2 RR activity of ClAg14 (C≡Ct Bu)12 + NCs are investigated by a combination of electrokinetic and theoretical studies. These analyses reveal that different active sites, generated through electrochemical activation, have unique adsorption properties for the reaction intermediates, leading to enhanced CO2 RR and suppressed hydrogen production. Furthermore, industrially relevant CO2 -to-CO electroreduction using ClAg14 (C≡Ct Bu)12 + NCs in a zero-gap CO2 electrolyzer, achieving high energy efficiency of 51% and catalyst activity of over 1400 A g-1 at a current density of 400 mA cm-2 is demonstrated.

Hydride-Containing Pt-doped Cu-rich Nanoclusters: Synthesis, Structure, and Electrocatalytic Hydrogen Evolution.

A structurally precise hydride-containing Pt-doped Cu-rich nanocluster [PtH2 Cu14 {S2 P(Oi Pr)2 }6 (CCPh)6 ] (1) has been synthesized. It consists of a bicapped icosahedral Cu14 cage that encapsulates a linear PtH2 unit. Upon the addition of two equivalents of CF3 COOH to 1, two hydrido clusters are isolated. These clusters are [PtHCu11 {S2 P(Oi Pr)2 }6 (CCPh)4 ] (2), which is a vertex-missing Cu11 cuboctahedron encaging a PtH moiety, and [PtH2 Cu11 {S2 P(Oi Pr)2 }6 (CCPh)3 ] (3), a distorted 3,3,4,4,4-pentacapped trigonal prismatic Cu11 cage enclosing a PtH2 unit. The electronic structure of 2, analyzed by Density Functional Theory, is a 2e superatom. The electrocatalytic activities of 1-3 for hydrogen evolution reaction (HER) were compared. Notably, Cluster 2 exhibited an exceptionally excellent HER activity within metal nanoclusters, with an onset potential of -0.03 V (at 10 mA cm-2 ), a Tafel slope of 39 mV dec-1 , and consistent HER activity throughout 3000 cycles in 0.5 M H2 SO4 . Our study suggests that the accessible central Pt site plays a crucial role in the remarkable HER activity and may provide valuable insights for establishing correlations between catalyst structure and HER activity.

Comparison of visual quantities in untrained neural networks.

The ability to compare quantities of visual objects with two distinct measures, proportion and difference, is observed even in newborn animals. However, how this function originates in the brain, even before visual experience, remains unknown. Here, we propose a model in which neuronal tuning for quantity comparisons can arise spontaneously in completely untrained neural circuits. Using a biologically inspired model neural network, we find that single units selective to proportions and differences between visual quantities emerge in randomly initialized feedforward wirings and that they enable the network to perform quantity comparison tasks. Notably, we find that two distinct tunings to proportion and difference originate from a random summation of monotonic, nonlinear neural activities and that a slight difference in the nonlinear response function determines the type of measure. Our results suggest that visual quantity comparisons are primitive types of functions that can emerge spontaneously before learning in young brains.

Superatom-in-Superatom Nanoclusters: Synthesis, Structure, and Photoluminescence.

We report a new strategy in which a thiolate-protected Ag25 nanocluster can be doped with open d-shell group 8 (Ru, Os) and 9 (Ir) metals by forming metal hydride (RuH2 , OsH2 , IrH) superatoms with a closed d-shell. Structural analyses using various experimental and theoretical methods revealed that the Ag25 nanoclusters were co-doped with the open d-shell metal and hydride species to produce superatom-in-superatom nanoclusters, establishing a novel superatom doping phenomenon for open d-shell metals. The synthesized superatom-in-superatom nanoclusters exhibited dopant-dependent photoluminescence (PL) properties. Comparative PL lifetime studies of the Ag25 nanoclusters doped with 8-10 group metals revealed that both radiative and nonradiative processes were significantly dependent on the dopant. The former is strongly correlated with the electron affinity of the metal dopant, whereas the latter is governed predominantly by the kernel structure changed upon the doping of the metal hydride(s).

Hydride-containing 2-Electron Pd/Cu Superatoms as Catalysts for Efficient Electrochemical Hydrogen Evolution.

The first hydride-containing 2-electron palladium/copper alloys, [PdHCu11 {S2 P(Oi Pr)2 }6 (C≡CPh)4 ] (PdHCu11 ) and [PdHCu12 {S2 P(Oi Pr)2 }5 {S2 PO(Oi Pr)} (C≡CPh)4 ] (PdHCu12 ), are synthesized from the reaction of [PdH2 Cu14 {S2 P(Oi Pr)2 }6 (C≡CPh)6 ] (PdH2 Cu14 ) with trifluoroacetic acid (TFA). X-ray diffraction reveals that the PdHCu11 and PdHCu12 kernels consist of a central PdH unit encapsulated within a vertex-missing Cu11 cuboctahedron and complete Cu12 cuboctahedron, respectively. DFT calculations indicate that both PdHCu11 and PdHCu12 can be considered as axially-distorted 2-electron superatoms. PdHCu11 shows excellent HER activity, unprecedented within metal nanoclusters, with an onset potential of -0.05 V (at 10 mA cm-2 ), a Tafel slope of 40 mV dec-1 , and consistent HER activity during 1000 cycles in 0.5 M H2 SO4 . Our study suggests that the accessible central Pd site is the key to HER activity and may provide guidelines for correlating catalyst structures and HER activity.

Transplanting Gold Active Sites into Non-Precious-Metal Nanoclusters for Efficient CO2-to-CO Electroreduction.

Electrocatalytic CO2 reduction reaction (CO2RR) is greatly facilitated by Au surfaces. However, large fractions of underlying Au atoms are generally unused during the catalytic reaction, which limits mass activity. Herein, we report a strategy for preparing efficient electrocatalysts with high mass activities by the atomic-level transplantation of Au active sites into a Ni4 nanocluster (NC). While the Ni4 NC exclusively produces H2, the Au-transplanted NC selectively produces CO over H2. The origin of the contrasting selectivity observed for this NC is investigated by combining operando and theoretical studies, which reveal that while the Ni sites are almost completely blocked by the CO intermediate in both NCs, the Au sites act as active sites for CO2-to-CO electroreduction. The Au-transplanted NC exhibits a remarkable turnover frequency and mass activity for CO production (206 molCO/molNC/s and 25,228 A/gAu, respectively, at an overpotential of 0.32 V) and high durability toward the CO2RR over 25 h.

Controlled syngas production by electrocatalytic CO2 reduction on formulated Au25(SR)18 and PtAu24(SR)18 nanoclusters.

Syngas, a gaseous mixture of CO and H2, is a critical industrial feedstock for producing bulk chemicals and synthetic fuels, and its production via direct CO2 electroreduction in aqueous media constitutes an important step toward carbon-negative technologies. Herein, we report controlled syngas production with various H2/CO ratios via the electrochemical CO2 reduction reaction (CO2RR) on specifically formulated Au25 and PtAu24 nanoclusters (NCs) with core-atom-controlled selectivities. While CO was predominantly produced from the CO2RR on the Au NCs, H2 production was favored on the PtAu24 NCs. Density functional theory calculations of the free energy profiles for the CO2RR and hydrogen evolution reaction (HER) indicated that the reaction energy for the conversion of CO2 to CO was much lower than that for the HER on the Au25 NC. In contrast, the energy profiles calculated for the HER indicated that the PtAu24 NCs have nearly thermoneutral binding properties; thus, H2 production is favored over CO formation. Based on the distinctly different catalytic selectivities of Au25 and PtAu24 NCs, controlled syngas production with H2/CO ratios of 1 to 4 was demonstrated at a constant applied potential by simply mixing the Au25 and PtAu24 NCs based on their intrinsic catalytic activities for the production of CO and H2.

Superatom-in-Superatom [RhH@Ag24 (SPhMe2 )18 ]2- Nanocluster.

Heterometal doping is a powerful method for tuning the physicochemical properties of metal nanoclusters. While the heterometals doped into such nanoclusters predominantly include transition metals with closed d-shells, the doping of open d-shell metals remains largely unexplored. Herein, we report the first synthesis of a [RhHAg24 (SPhMe2 )18 ]2- nanocluster, in which a Rh atom with open d-shells ([Kr]4d8 5s1 ) is incorporated into the Ag24 framework by forming a RhH superatom with closed d-shells ([Kr]4d10 ). Combined experimental and theoretical investigations showed that the Ag24 framework was co-doped with Rh and hydride and that the RhH dopant was a superatomic construct of a Pd atom. Additional studies demonstrated that the [RhHAg24 (SPhMe2 )18 ]2- nanocluster was isoelectronic to the [PdAg24 (SPhMe2 )18 ]2- nanocluster with the superatomic 8-electron configuration (1S2 1P6 ). This study demonstrated for the first time that a superatom could be incorporated into a cluster superatom to generate a stable superatom-in-superatom nanocluster.

Atomically Precise Gold Nanoclusters as Model Catalysts for Identifying Active Sites for Electroreduction of CO2.

Accurate identification of active sites is critical for elucidating catalytic reaction mechanisms and developing highly efficient and selective electrocatalysts. Herein, we report the atomic-level identification of active sites using atomically well-defined gold nanoclusters (Au NCs) Au25 , Au38 , and Au144 as model catalysts in the electrochemical CO2 reduction reaction (CO2 RR). The studied Au NCs exhibited remarkably high CO2 RR activity, which increased with increasing NC size. Electrochemical and X-ray photoelectron spectroscopy analyses revealed that the Au NCs were activated by removing one thiolate group from each staple motif at the beginning of CO2 RR. In addition, density functional theory calculations revealed higher charge densities and upshifts of d-states for dethiolated Au sites. The structure-activity properties of the studied Au NCs confirmed that dethiolated Au sites were the active sites and that CO2 RR activity was determined by the number of active sites on the cluster surface.

Synthesis and Photophysical Properties of Light-Harvesting Gold Nanoclusters Fully Functionalized with Antenna Chromophores.

The development of efficient light-harvesting systems is important to understand the key aspects of solar-energy conversion processes and to utilize them in various photonic applications. Here, atomically well-defined gold nanoclusters are reported as a new platform to fabricate artificial light-harvesting systems. An efficient amide coupling method is developed to synthesize water-soluble Au22 clusters fully protected with pyrene chromophores by taking advantage of their facile phase-transfer reaction. The synthesized Au22 clusters with densely packed 18 pyrene chromophores (Au22 -PyB18 ) exhibit triple-emission in blue, green, and red wavelength regions arising respectively from pyrene monomer, pyrene excimer, and Au22 emission, producing bright white light emission together. The photoluminescence of Au22 is enhanced by more than tenfold, demonstrating that pyrenes at the periphery efficiently channel the absorbed energy to the luminescent Au22 at the center. A combination of femtosecond transient absorption and anisotropy measurements of Au22 -PyB18 explicitly reveals three main decay components of 220 fs, 3.5 ps, and 160 ps that can be assigned to energy migration between pyrenes and energy transfer processes from pyrene monomer and excimer to the central Au22 , respectively.

Energy band alignment at the heterointerface between a nanostructured TiO2 layer and Au22(SG)18 clusters: relevance to metal-cluster-sensitized solar cells.

This study is the first to quantify energy band alignments at a nanostructured TiO2/Au22(SG)18 cluster interface using X-ray photoelectron spectroscopy. The d-band of Au clusters shows band-like character and occupied states at the Fermi level are not detected. The results provide evidence of the existence of a finite optical energy gap in Au22(SG)18 clusters and the molecular-like nature of these clusters. The pinning position of the Fermi energy level at the interface was determined to be 2.8 and 1.3 eV higher than the top of the TiO2 valence band and the highest occupied molecular orbit level of the Au clusters, respectively. A diffuse reflectance and absorption analysis quantified a 3.2 eV bandgap of the TiO2 layer and a 2.2 eV energy gap between the highest occupied molecular orbit (HOMO) and the lowest unoccupied molecular orbit (LUMO) levels of the Au clusters. Thus, a cliff-like offset of 0.5 eV between the LUMO level and the TiO2 conduction band was determined. The cliff-like offset of 0.5 eV provides room for improving the efficiency of metal-cluster-sensitized solar cells (MCSSC) further by lowering the LUMO level through a change in the cluster size. The offset of 0.5 eV between the HOMO level and the 3I-/I-3 redox level yields a remarkable loss-in-potential, which implies the possibility of increasing the open-circuit voltage further by properly replacing the redox couple in the MCSSCs.

All 3D-Printed Flexible ZnO UV Photodetector on an Ultraflat Substrate.

An all three-dimensional (3D)-printed flexible ZnO ultraviolet (UV) photodetector is demonstrated, where the 3D-printing method is used not only for the electrode and photosensitive material but also for creating a substrate. An ultraflat and flexible substrate capable of serving as the backbone layer is developed using a water-dissolvable polymer layer for surface planarization. A two-layered printing followed by surface treatment is demonstrated for the substrate preparation. As mechanical support but flexible, a thick and sparse thermoplastic polyurethane layer is printed. On its surface, a thin and dense poly(vinyl alcohol) (PVA) is then printed. A precise control of PVA reflow using a microwater droplet results in a flexible and extremely uniform substrate. A Cu-Ag nanowire network is directly 3D printed on the flexible substrate for the conducting layer, followed by ZnO for the photosensitive material. Unlike the planar two-dimensional printing that provides thin films, 3D printing allows the electrode to have a step height, which can be made like a dam to accommodate a thick film of ZnO. Photosensitivity as a function of various ZnO thickness values was investigated to establish an optimal thickness for UV response. The device was also tested in natural sunlight along with stability and reliability.

On-Demand Printing of Wearable Thermotherapy Pad.

Thermotherapy is an effective method for pain relief, recovery from injury, and general healthcare. The ordinary heat pad used for thermotherapy at home is not usually tailored to the individual but supplied in a few different pre-fixed sizes and shapes for mass marketing. A customized wearable heat pad often requires expert support. Herein, an instant, custom-fit, and on-demand heat pad for thermotherapy is demonstrated. The heater is directly printed using silver nanoparticle ink on an off-the-shelf medical grade tape by inkjet technology. By coating the tape with silica nanoparticles as ink-absorbing layer and chloride ions as chemical sintering agent, stable heater patterns are printed without the need for subsequent high temperature sintering process. A 3D scanner is used to acquire body information, and a customized heater is produced using the information. The printed heat pad is attached to the shoulder and the effect of thermotherapy is verified objectively through electroencephalography and subjectively through survey. This printed heat pad produced by simple and low-cost fabrication provides wearable medical devices for personal thermotherapy.

Elucidating ligand effects in thiolate-protected metal clusters using Au24Pt(TBBT)18 as a model cluster.

2-Phenylethanethiolate (PET) and 4-tert-butylbenzenethiolate (TBBT) are the most frequently used ligands in the study of thiolate (SR)-protected metal clusters. However, the effect of difference in the functional group between these ligands on the fundamental properties of the clusters has not been clarified. We synthesized [Au24Pt(TBBT)18]0, which has the same number of metal atoms, number of ligands, and framework structure as [Au24Pt(PET)18]0, by replacing ligands of [Au24Pt(PET)18]0 with TBBT. It was found that this ligand exchange is reversible unlike the case of other metal-core clusters. A comparison of the geometrical/electronic structure and stability of the clusters between [Au24Pt(PET)18]0 and [Au24Pt(TBBT)18]0 revealed three things with regard to the effect of ligand change from PET to TBBT on [Au24Pt(SR)18]0: (1) the induction of metal-core contraction and Au-S bond elongation, (2) no substantial effect on the HOMO-LUMO gap but a clear difference in optical absorption in the visible region, and (3) the decrease of stabilities against degradation in solution and under laser irradiation. By using these two clusters as model clusters, it is expected that the effects of the structural difference of ligand functional-groups on the physical properties and functions of clusters, such as catalytic ability and photoluminescence, would be clarified.

Electrochemistry of Atomically Precise Metal Nanoclusters.

Thiolate-protected metal nanoparticles containing a few to few hundred metal atoms are interesting materials exhibiting unique physicochemical properties. They encompass the bulk-to-molecule transition region, where discrete electronic states emerge and electronic band energetics yield to quantum confinement effects. Recent progresses in the synthesis and characterization of ultrasmall gold nanoparticles have opened up new avenues for the isolation of extremely monodispersed nanoparticles with atomically precision. These nanoparticles are also called nanoclusters to distinguish them from other regular metal nanoparticles with core diameter >2 nm. These nanoclusters are typically identified by their actual molecular formulas; prominent among these are Au25(SR)18, Au38(SR)24, and Au102(SR)44, where SR is organothiolate. A number of single crystal structures of these nanoclusters have been disclosed. Researchers have effectively utilized density functional theory (DFT) calculations to predict their atomic and electronic structures, as well as their physicochemical properties. The atomically precise metal nanoclusters have been the focus of recent studies owing to their novel size-specific electrochemical, optical, and catalytic properties. In this Account, we highlight recent advances in electrochemistry of atomically precise metal nanoclusters and their applications in electrocatalysis and electrochemical sensing. Compared with gold nanoclusters, much less progress has been made in the electrochemical studies of other metal nanoclusters, and thus, we mainly focus on the electrochemistry and electrochemical applications of gold-based nanoclusters. Voltammetry has been extremely powerful in investigating the electronic structure of metal nanoclusters, especially near HOMO and LUMO levels. A sizable opening of HOMO-LUMO gap observed for Au25(SR)18 gradually decreases with increasing nanocluster size, which is in line with the change in the optical gap. Heteroatom-doping has been a powerful strategy to modify the optical and electrochemical properties of metal nanoclusters at the atomic level. While the superatom theory predicts 8-electron configuration for [Au25(SR)18]- and many doped nanoclusters thereof, Pt- and Pd-doped [PtAu24(SR)18]0 and [PdAu24(SR)18]0 nanoclusters show dramatically different electronic structures, as manifested in their optical spectra and voltammograms, suggesting the occurrence of the Jahn-Teller distortion in these doped nanoclusters. Furthermore, metal-doping may alter their surface binding properties, as well as redox potentials. Metal nanoclusters offer great potential for attaining high activity and selectivity in their electrocatalytic applications. The well-defined core-shell structure of a metal nanocluster is of special advantage because the core and shell can be independently engineered to exhibit suitable binding properties and redox potentials. We discuss recent progress made in electrocatalysis based upon metal nanoclusters tailored for water splitting, CO2 conversion, and electrochemical sensing. A well-defined model nanocatalyst is absolutely necessary to reveal the detailed mechanism of electrocatalysis and thereby to lead to the development of a new efficient electrocatalyst. We envision that atomically controlled metal nanoclusters will enable us to systematically optimize the electrochemical and surface properties suitable for electrocatalysis, thus providing a powerful platform for the discovery of finely tuned nanocatalysts.

Effects of Metal-Doping on Hydrogen Evolution Reaction Catalyzed by MAu24 and M2Au36 Nanoclusters (M = Pt, Pd).

This paper describes the effects of doped metals on hydrogen evolution reaction (HER) electrocatalyzed by atomically controlled MAu24 and M2Au36 nanoclusters, where M = Pt and Pd. HER performances, such as onset potential ( Eonset), catalytic current density, and turnover frequency (TOF), are comparatively examined with respect to the doped metals. Doping Pt or Pd into gold nanoclusters not only changes the electrochemical redox potentials of nanoclusters but also considerably improves the HER activities. Eonset is found to be controlled by the nanocluster's reduction potential matching the reduction potential of H+. The higher catalytic current and TOF are observed with the doped nanoclusters in the order of PtAu24 > PdAu24 > Au25. The same trend is observed with the Au38 group (Pt2Au36 > Pd2Au36> Au38). Density functional theory calculations have revealed that the hydrogen adsorption free energy (Δ GH) is significantly lowered by metal-doping in the order of Au25 > PdAu24 > PtAu24 and Au38 > Pd2Au36 > Pt2Au36, indicating that hydrogen adsorption on the active site of nanocluster is thermodynamically favored by Pd-doping and further by Pt-doping. The doped metals, albeit buried in the core of the nanoclusters, have profound impact on their HER activities by altering their reduction potentials and hydrogen adsorption free energies.

Unique Energy Transfer in Fluorescein-Conjugated Au22 Nanoclusters Leading to 160-Fold pH-Contrasting Photoluminescence.

Accurate measurements of intracellular pH are of crucial importance in understanding the cellular activities and in the development of intracellular drug delivery systems. Here we report a highly sensitive pH probe based on a fluorescein-conjugated Au22 nanocluster. Steady-state photoluminescence (PL) measurements have shown that, when conjugated to Au22, fluorescein exhibits more than 160-fold pH-contrasting PL in the pH range of 4.3-7.8. Transient absorption measurements show that there are two competing ultrafast processes in the fluorescein-conjugated Au22 nanocluster: the intracore-state relaxation and the energy transfer from the nonthermalized states of Au22 to fluorescein. The latter becomes predominant at a higher pH, leading to dramatic PL enhancement of fluorescein. In addition to the intrinsically low toxicity, fluorescein-conjugated Au22 nanoclusters exhibit high pH sensitivity, wide dynamic range, and excellent photostability, providing a powerful tool for the study of intracellular processes.