Thiolate-protected Gold Nanoclusters Research Articles

Three-Stage Evolution from Nonscalable to Scalable Optical Properties of Thiolate-Protected Gold Nanoclusters.

The evolution of the optical properties of gold nanoclusters (NCs) versus size is of great importance because it not only reveals the nature of quantum confinement in NCs, but also helps to understand how the molecular-like Au NCs transit to plasmonic nanoparticles. While some work has been done in studying the optical properties of NCs of certain individual sizes, the global picture remains unclear, such as the detailed relationship between size/structure and properties. Here, we investigate the grand evolution of the optical properties by comparing the steady-state absorption, bandgap, transient absorption, as well as carrier dynamics of a series of thiolate-protected gold NCs ranging from tens to hundreds of gold atoms. We find that, on the basis of their optical behaviors, gold NCs can be classified into three groups: (i) ultrasmall NCs (ca. <50 Au atoms) are nonscalable as their optical properties are strongly dependent on the structure rather than size; (ii) medium-sized NCs (about 50-100 Au atoms) show both size- and structure-dependent optical properties; and (iii) large-sized gold NCs (ca. >100 Au atoms) exhibit optical properties solely dependent on size, and the structure effect fades out. Unraveling the grand evolution from nonscalable to scalable optical properties and their mechanisms will greatly deepen scientific understanding of the nature of quantum-sized gold NCs and will also provide implications for plasmonic NPs.

Vibrational Spectra of Thiolate-Protected Gold Nanocluster with Infrared Reflection Absorption Spectroscopy: Size- and Temperature-Dependent Ordering Behavior of Organic Monolayer

Vibrational spectra of thiolate-protected gold nanoclusters, prepared in a monolayer manner using the Langmuir–Blodgett method, were measured by means of infrared reflection absorption spectroscopy...

Single-atom doping on thiolate-protected gold nanoclusters: a TDDFT study on the excited states

The optical features of thiolate-protected gold nanoclusters can be tuned in different ways to achieve the desired features. One promising approach is doping with a single metallic atom to the gold core to form the ‘Atomically Precise Gold Nanoparticle’. However, the tuning mechanism and the changes in the excited states are not discussed thoroughly. In this work, we perform theoretical analyses of Ag, Cu, Pd, and Pt doped gold nanoclusters and focus on the doping induced changes in the excitations. Our results indicate several significant impacts of the platinum family dopings, which are not presented in the gold family dopings: low energy (infrared) excitation features, extra ‘in-gap’ energy levels, and significant redistribution of the electrons/holes in response to the external electric field.

Chiral Inversion of Thiolate-Protected Gold Nanoclusters via Core Reconstruction without Breaking a Au-S Bond.

On the basis of density functional theory computations of the well-known chiral Au38(SR)24 nanocluster and its Pd- and Ag-doped derivatives, we propose here a mechanism for chiral inversion that does not require the breaking of a metal–sulfur bond at the metal–ligand interface but features a collective rotation of the gold core. The calculated energy barriers for this mechanism for Au38 and Pd-doped Au38 are in the range of 1–1.5 eV, significantly lower than barriers involving the breakage of Au–S bonds (2.5 eV). For Ag-doped Au38, barriers for both mechanisms are similar (1.3–1.5 eV). Inversion barriers for a larger chiral Au144(SR)60 are much higher (2.5−2.8 eV). Our computed barriers are in good agreement with racemization barriers estimated from existing experiments for bare and doped Au38. These results highlight the sensitivity of chiral inversion to the size, structure, and metal composition of the metal core and sensitivity to the detailed structure of the metal–thiolate interface. Our work also predicts that enantiopure Au144(SR)60 clusters would be promising materials for applications requiring high resistance to chiral inversion.

A Mono-cuboctahedral Series of Gold Nanoclusters: Photoluminescence Origin, Large Enhancement, Wide Tunability, and Structure-Property Correlation.

The origin of the near-infrared photoluminescence (PL) from thiolate-protected gold nanoclusters (Au NCs, <2 nm) has long been controversial, and the exact mechanism for the enhancement of quantum yield (QY) in many works remains elusive. Meanwhile, based upon the sole steady-state PL analysis, it is still a major challenge for researchers to map out a definitive relationship between the atomic structure and the PL property and understand how the Au(0) kernel and Au(I)-S surface contribute to the PL of Au NCs. Herein, we provide a paradigm study to address the above critical issues. By using a correlated series of "mono-cuboctahedral kernel" Au NCs and combined analyses of steady-state, temperature-dependence, femtosecond transient absorption, and Stark spectroscopy measurements, we have explicitly mapped out a kernel-origin mechanism and clearly elucidate the surface-structure effect, which establishes a definitive atomic-level structure-emission relationship. A ∼100-fold enhancement of QY is realized via suppression of two effects: (i) the ultrafast kernel relaxation and (ii) the surface vibrations. The new insights into the PL origin, QY enhancement, wavelength tunability, and structure-property relationship constitute a major step toward the fundamental understanding and structural-tailoring-based modulation and enhancement of PL from Au NCs.

Structure-property relationships on thiolate-protected gold nanoclusters.

Since their discovery, thiolate-protected gold nanoclusters (Aun(SR)m) have garnered a lot of interest due to their fascinating properties and “magic-number” stability. However, models describing the thermodynamic stability and electronic properties of these nanostructures as a function of their size are missing in the literature. Herein, we employ first principles calculations to rationalize the stability of fifteen experimentally determined gold nanoclusters in conjunction with a recently developed thermodynamic stability theory on small Au nanoclusters (≤102 Au atoms). Our results demonstrate that the thermodynamic stability theory can capture the stability of large, atomically precise nanoclusters, Au279(SR)84, Au246(SR)80, and Au146(SR)57, suggesting its applicability over larger cluster size regimes than its original development. Importantly, we develop structure–property relationships on Au nanoclusters, connecting their ionization potential and electron affinity to the number of gold atoms within the nanocluster. Altogether, a computational scheme is described that can aid experimental efforts towards a property-specific, targeted synthesis of gold nanoclusters.

Growth-Rule-Guided Structural Exploration of Thiolate-Protected Gold Nanoclusters.

Understanding the structure and structure-property relationship of atomic and ligated clusters is one of the central research tasks in the field of cluster research. In chemistry, empirical rules such as the polyhedral skeleton electron pair theory (PSEPT) approach had been outlined to account for skeleton structures of many main-group atomic and ligand-protected transition metal clusters. Nonetheless, because of the diversity of cluster structures and compositions, no uniform structural and electronic rule is available for various cluster compounds. Exploring new cluster structures and their evolution is a hot topic in the field of cluster research for both experiment and theory. In this Account, we introduce our recent progress in the theoretical exploration of structures and evolution patterns of a class of atomically precise thiolate-protected gold nanoclusters using density functional theory computations. Unlike the conventional ligand-protected transition metal compounds, the thiolate-protected gold clusters demonstrate novel metal core/ligand shell interfacial structures in which the Au m(SR) n clusters can be divided into an ordered Au(0) core and a group of oligomeric SR[Au(SR)] x ( x = 0, 1, 2, 3, ...) protection motifs. Guided by this "inherent structure rule", we have devised theoretical methods to rapidly explore cluster structures that do not necessarily require laborious global potential energy surface searches. The structural predictions of Au38(SR)24, Au24(SR)20, and Au44(SR)28 nanoclusters were completely or partially verified by the later X-ray crystallography studies. On the basis of the analysis of cluster structures determined by X-ray crystallography and theoretical prediction, a structural evolution diagram for the face-centered-cubic (fcc)-type Au m(SR) n clusters with m up to 92 has been preliminarily established. The structural evolution diagram indicates some basic structural and electronic evolution patterns of thiolate-protected gold nanoclusters. The fcc Au m(SR) n clusters show a genetic structural evolution pattern in which each step of cluster size increase results in the formation of another Au4 tetrahedron or Au3 triangle unit in the Au core, and every increase of a structural unit in the Au core leads to an increase of two electrons in the whole cluster. The unique one- or two-dimensional cluster size evolution, the isomerism of the Au-S framework, and the formation of a double-helical and cyclic tetrahedron network in the fcc Au m(SR) n clusters all can be addressed from this evolution pattern. The summarized cluster structural evolution diagrams enable us to further explore more stable cluster structures and understand their structure-electronic structure-property relationships.

Insights into Interfaces, Stability, Electronic Properties, and Catalytic Activities of Atomically Precise Metal Nanoclusters from First Principles.

Atomically precise, ligand-protected metal nanoclusters are of great interest for their well-defined structures, intriguing physicochemical properties, and potential applications in catalysis, biology, and nanotechnology. Their structure precision provides many opportunities to correlate their geometries, stability, electronic properties, and catalytic activities by closely integrating theory and experiment. In this Account, we highlight recent theoretical advances from our efforts to understand the metal-ligand interfaces, the energy landscape, the electronic structure and optical absorption, and the catalytic applications of atomically precise metal nanoclusters. We mainly focus on gold nanoclusters. The bonding motifs and energetics at the gold-ligand interfaces are two main interests from a computational perspective. For the gold-thiolate interface, the -RS-Au-SR- staple motif is not always preferred; in fact, the bridging motif (-SR-) is preferred at the more open facets such as Au(100) and Au(110). This finding helps understand the diversity of the gold-thiolate motifs for different core geometries and sizes. A great similarity is demonstrated between gold-thiolate and gold-alkynyl interfaces, regarding formation of the staple-type motifs with PhC≡C- as an example. In addition, N-heterocyclic carbenes (NHCs) without bulky groups also form the staple-type motif. Alkynyls and bulky NHCs have the strongest binding with the gold surface from comparing 27 ligands of six types, suggesting a potential to synthesize NHC-protected gold clusters. The energy landscape of nanosystems is usually complex, but experimental progress in synthesizing clusters of the same Au-S composition with different R groups and isomers of the same Au n(SR) m formula have made detailed theoretical analyses of energetic contributions possible. Ligand-ligand interactions turn out to play an important role in the cluster stability, while metastable isomers can be obtained via kinetic control. Although the superatom-complex theory is the starting point to understand the electronic structure of atomically precise gold clusters, other factors also greatly affect the orbital levels that manifest themselves in the experimental optical absorption spectra. For example, spin-orbit coupling needs to be included to reproduce the splitting of the HOMO-LUMO transition observed experimentally for Au25(SR)18-, the poster child of the family. In addition, doping can lead to structural changes and charge states that do not follow the superatomic electron count. Atomically precise metal nanoclusters are an ideal system for understanding nanocatalysis due to their well-defined structures. Active sites and catalytic mechanisms are explored for selective hydrogenation and hydrogen evolution on thiolate-protected gold nanoclusters with and without dopants. The behavior of H in nanogold is analyzed in detail, and the most promising site to attract H is found to be coordinately unsaturated Au atoms. Many insights have been gained from first-principles studies of atomically precise, ligand-protected gold nanoclusters. Interesting and important questions remaining to be addressed are pointed out in the end.

Vibrational Properties of Thiolate-Protected Gold Nanoclusters.

Over recent years, the field of thiolate-protected gold nanoclusters has made remarkable progress. The successful determination of the structure of some of these clusters by X-ray crystallography was a milestone in this field. X-ray crystallography is arguably the most important technique in the field up to now, and it enabled the study of structure evolution as a function of cluster size. It also shed light on the structure of the Au-S interface. Recently, it has been realized that thiolate-protected gold clusters are very dynamic systems. Metal atoms and ligands can exchange easily between clusters. Furthermore, the adsorbed ligands bear conformational dynamics. Such dynamic effects call for experimental methods that can cope with it. Future efforts in this field will be directed toward applications of thiolate-protected clusters, and many of them will rely on dissolved clusters. Therefore, structure determination in solution is an important issue, though it is very challenging. The structure of the metal core and the Au-S interface is not expected to change in solution with respect to the crystal. However, the structure of the adsorbed ligand itself is sensitive to the environment and may be different in the solid state and in solution, as has been shown in fact in the past. It is this (dynamic) structure of the ligand that determines the interaction between the cluster and its environment, which is crucial, for example, for sensing applications. Vibrational spectroscopy is a promising technique to characterize thiolate-protected clusters in different environments. A vibrational spectrum is sensitive to structure (conformation) although this information is often "hidden" in the spectrum, requiring detailed analysis and support from theory to be deciphered. Compared to other techniques like UV-vis spectroscopy and mass spectrometry, vibrational spectroscopy was not extensively used in the field of thiolate-protected clusters, but we believe that the technique will be very valuable for the future developments in the field. We have used vibrational spectroscopy to investigate thiolate-protected gold clusters for mainly two lines of research. In the first, we studied in detail the low energy region of the vibrational spectrum, in particular the Au-S vibrational modes, in order to understand the structure sensitivity. It emerges that the Au-S vibrational spectrum is indeed sensitive to the structure of the interface but also to other factors, especially the organic part of the thiol, in a complex way. The ability to directly correlate structure, from X-ray crystallography, and vibrational spectra for thiolate-protected clusters, should lead to a database that will help in the future the structure determination of the Au-S interface by vibrational spectroscopy for systems where direct structure determination is not possible, for example, for flat surfaces. A second line of research focused on the determination of the structure of the adsorbed ligands for dissolved clusters. Such information is mostly extracted by the comparison of theoretical and calculated spectra for different conformers. In this respect, vibrational circular dichroism (VCD) is particularly powerful as it strongly depends on the conformation, more than conventional infrared spectroscopy. VCD can be applied to chiral nonracemic compounds, and it is a sensitive probe for chirality. Using this method, it was possible to demonstrate that a cluster can transfer its chirality to achiral thiolate ligands. In this Account, we summarize the possibilities and challenges of vibrational spectroscopy in the field of thiolate-protected clusters.

Core Geometry Effect on the Bonding Properties of Gold–Thiolate Nanoclusters: The Case of Hexagonal-Close-Packed Au30(SR)18

Diversity of core geometry for thiolate-protected gold nanoclusters (AuSR NCs) is a fascinating phenomenon, and thus, understanding the core geometry effect on the bonding properties of AuSR NCs is...

Alkynyl Approach toward the Protection of Metal Nanoclusters.

The past decades have witnessed great advances in the synthesis, structure determination, and properties investigation of coinage metal nanoclusters. These monodisperse clusters have well-defined molecular structures, which is advantageous in correlating structures and properties. Metal nanoclusters are large molecules consisting of many components, so it is a big challenge to prepare them in a rational way. Strenuous efforts have been madeto control their geometric and electronic structures, in order to optimize their various properties. A metal nanocluster normally contains a metal core and a peripheral ligand shell. The ligands do not only function as simple stabilizing agents. It has been revealed that these ligands are able to influence the formation processes of the nanoclusters, and they may also dictate the sizes, shapes, and properties of nanoclusters. There are mainly three types of ligands that are widely used as surface anchors on coinage metal nanoclusters: thiolates, phosphines, and halides. Recent ligand engineering has extended the scope to alkynyl ligands. As alkynyl ligands are versatile in interacting with metal atoms, interesting alkynyl-metal interfacial structures including linear, L-shaped, and V-shaped staple motifs can be generated, as well as a series of novel coinage metal nanoclusters that exhibit intriguing molecular geometries. The staple motifs do not simply resemble the surface structures of thiolate-protected nanoclusters, because the incorporation of alkynyl ligands may significantly alter diverse properties of nanoclusters. Compared with thiolate-protected gold nanoclusters, alkynyl-protected ones with identical metal cores exhibit distinctly different absorption profiles and show much improved catalytic activities for semihydrogenation of alkynes. In addition, the participation of alkynyl ligands could profoundly affect the luminescent properties of nanoclusters. These "ligand effects" are mainly attributed to the different nature of alkynyl ligands, as electronic perturbation through π-conjugated units may largely modulate the electronic structure of the whole cluster. In this Account, we describe the development of coinage metal nanoclusters protected with alkynyl ligands. We will first briefly bring up the emergence of alkynyl ligands as anchoring groups on the surfaces of nanoclusters. Then we present the direct reduction method for the synthesis of the following four categories of nanoclusters: (a) gold nanoclusters with mixed-ligand shells, (b) all alkynyl-protected gold nanoclusters, (c) heterobimetallic gold nanoclusters, and (d) silver nanoclusters. Their molecular structures are described, and their various alkynyl-metal interfacial structures are compared with thiolate-metal staples. Finally, ligand effects on the properties of the clusters, including optical absorption, luminescence, and catalysis, are discussed. The alkynyl ligands play an important role in terms of both structural and property aspects. We believe this Account will attract increasing attention to alkynyl ligands, which have shown promising potential in generating new structures and properties of coinage metal nanoclusters.

Precision at the nanoscale: on the structure and property evolution of gold nanoclusters

Abstract Chemists are often regarded as “architects”, who are capable of building up complex molecular structures in the ultrasmall-dimensional world. However, compared with organic chemistry, nanochemistry – which deals with nanoparticles in the size range from 1 to 100 nm – is less precise in terms of synthesis, composition, and structure. Such an imprecise nature of nanochemistry has impeded an in-depth understanding as well as rational control of structures and properties of nanomaterials. Motivated by this, thiolate-protected gold nanoclusters (denoted as Au n (SR) m ) have recently emerged as a paradigm of atomically precise nanomaterials, in which all the nanoparticles are identical to each other with the same number of core atoms (n) and surface ligands (m) as well as the atomic arrangement. In this review, we provide a demonstration of how the precise nature of Au n (SR) m nanoclusters allows one to understand, decipher and discover some important, enigmatic and intriguing issues and phenomena in nanoscience, including (i) a precise nanoscale transformation reaction induced by surface ligand exchange, (ii) the total structures of crystalline metal phases and the self-assembled surface monolayers, (iii) the periodicities and quantum confinement in nanoclusters and (iv) the emergence of hierarchical complexity in the entire nanoparticle system. We expect that such an in-depth understanding will eventually lead to the rational design and precise engineering of complex architectures at the nanoscale.

Theoretical Investigation of Relaxation Dynamics in Au38(SH)24 Thiolate-Protected Gold Nanoclusters

A subtle change in the electronic structure of thiolate-protected noble metal nanoparticles can result in distinctive energy relaxation dynamics. Corresponding investigations on different sizes and structures of thiolate-protected gold nanoclusters reveal their physical and chemical properties for further development of catalytic applications. In this work, we performed nonradiative relaxation dynamics simulations of the Au38(SH)24 nanocluster to describe electron-vibrational energy exchange. The core and higher excited states involving semiring motifs lying in the energy range of 0.00–2.01 eV are investigated using time-dependent density functional theory (TDDFT). The surface hopping method with decoherence correction combined with real-time TDDFT is used to assess the quantum dynamics. The Au23 core relaxations are found to occur in the range of 2.0–8.2 ps. The higher excited states that consist of core–semiring mixed or semiring states give ultrafast decay time constants in the range of 0.6–4.9 ps. Our...

Structure and Electronic Structure Evolution of Thiolate-Protected Gold Nanoclusters Containing Quasi Face-Centered-Cubic Kernels

A structure evolution map of face-centered cubic (fcc)-structured thiolate-ligand protected gold nanoclusters is outlined on the basis of total structure determination of a new 6e Au21(SR)15 (R = tert-butyl, t-Bu) cluster. The structural evolution map described some basic structural evolution patterns such as a triangle-Au3 and tetrahedron-Au4 associated gold-core evolution pattern and the periodic or symmetric growth of gold cores and ligand shells. According to the structural evolution map, a topological structure–electronic structure relationship is also proposed. The delocalized valence electronic properties of any fcc-structured gold clusters may be expressed as the linear combinations of the molecular orbitals of the fragment 2e units (Au3+ and Au42+). The structural disciplines and topological structure–electronic structure relationships reported in this work laid a basis for understanding the structural evolution and electronic structure of fcc-structured thiolate-protected gold nanoclusters. Part...

Characterization of floating-gate memory device with thiolate-protected gold and gold-palladium nanoclusters

The floating-gate memory characteristics of thiolate-protected gold (Au:SR) and palladium doped Au (AuPd:SR) nanoclusters, Au25(SR)18, Au24Pd(SR)18, and Au38(SR)24 (R = C12H25), were investigated by capacitance-voltage (C–V) measurements in vacuum. Monolayer films of Au:SR nanoclusters were formed as floating-gate memory layers on p-type Si substrates by the Langmuir-Schaefer method with surface pressure − area (π-A) isotherm measurements. A fluoropolymer (CYTOP, ∼15 nm thick) was spin-coated on top to form a hydrophobic insulating layer. Using an Au pad (∼40 nm thick) as the gate electrode, C–V measurements exhibit clockwise hysteresis curves originating from the Au:SR and AuPd:SR nanoclusters against the reference measured in each sample, and the hysteresis widths were dependent on the composition and sizes of the Au:SR nanoclusters. The positive and negative voltage shifts in the hysteresis can be explained in terms of electronic structures in Au:SR and AuPd:SR-based devices.

Aggregation/Self-Assembly-Induced Approach for Efficient AuAg Bimetallic Nanocluster-Based Photosensitizers

Great efforts have been devoted to the exploration of potential diagnostic and therapeutic applications of thiolate-protected gold nanoclusters (Au NCs). One of the therapeutic applications is the photosensitized generation of highly reactive singlet oxygen (1O2) using Au NCs for photodynamic therapy. However, there is scope for improving the 1O2-generation efficiency of Au NC photosensitizers. In this study, we exploit three strategies to improve the 1O2-generation efficiency of glutathione-protected Au NCs: (i) doping with silver (i.e., using bimetallic AuAg NCs, (ii) achieving aggregation-induced emission (AIE) using a Au(I)–thiolate complex, and (iii) achieving self-assembly-induced emission (SAIE) using a polymer nanogel. The combination of these three effects dramatically increased the 1O2-generation efficiency and enhanced the luminescence of the glutathione-protected Au NCs, owing to the inhibition of the nonradiative decay pathways. Finally, the photosensitizers based on AuAg NC@nanogel composite...

Origin of Photoluminescence of Ag25(SR)18– Nanoparticles: Ligand and Doping Effect

Recent theoretical insights into the origin of photoluminescence of thiolate-protected gold nanoclusters raise the question of whether the observed luminescence mechanism is valid for related silver nanoparticles. To this end, we perform density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations on the Ag25(SR)18– (R = H, PhMe2) nanocluster, which is currently the only thiolate-protected silver cluster that has a matching analogue in gold. The geometric and electronic structural modifications of Ag25(SH)18– upon photoexcitation are found to be similar but less pronounced than those of Au25(SH)18– at the same level of theory. The Stokes shift is calculated to be 0.37 eV and the replacement of R = H model ligands by R = PhMe2 decreases the Stokes shift in contrast to the increase in Stokes shift for aliphatic ligands previously observed in the Au25 system. The calculated emission energy agrees well with the experimental photoluminescence energy when the typical underes...

PH-Induced transformation of ligated Au25 to brighter Au23 nanoclusters.

Thiolate-protected gold nanoclusters have recently attracted considerable attention due to their size-dependent luminescence characterized by a long lifetime and large Stokes shift. However, the optimization of nanocluster properties such as the luminescence quantum yield is still a challenge. We report here the transformation of Au25Capt18 (Capt labels captopril) nanoclusters occurring at low pH and yielding a product with a much increased luminescence quantum yield which we have identified as Au23Capt17. We applied a simple method of treatment with HCl to accomplish this transformation and we characterized the absorption and emission of the newly created ligated nanoclusters as well as their morphology. Based on DFT calculations we show which Au nanocluster size transformations can lead to highly luminescent species such as Au23Capt17.

The structural isomerism in gold nanoclusters.

The isomerism in thiolate-protected gold (Au) nanoclusters is important for the understanding of structure-property correlations and the design of Au nanoclusters with specific structures and properties. Although recent studies have identified stereoisomerism, the understanding of structural isomerism is still lacking. Herein, we identified three distinct mechanisms of structural isomerism (i.e., core isomerism, staple isomerism, and complex isomerism) based on the crystallized isomers of thiolate-protected Au nanoclusters, and these mechanisms can be viewed as analogous to those of the structural isomerism in organic molecules (i.e., chain isomerism, point isomerism, and functional isomerism). Using the discovered core isomerism and staple isomerism, two Au28(SR)20 isomers are predicted and their synthesis feasibilities are illuminated. These new insights into the structural isomerism can facilitate rational design of new isomers of thiolate-protected Au nanoclusters and guide future experimental synthesis.

Label-free fluorescence turn-on strategy for trypsin activity based on thiolate-protected gold nanoclusters with bovine serum albumin as the substrate

By using bovine serum albumin (BSA) as the enzyme substrate, we propose a label-free, convenient, reliable and highly sensitive fluorescence “turn-on” assay for trypsin activity and inhibitor screening by means of fluorescent gold nanoclusters capped with 11-mercaptoundecanoic acid (namely AuNCs@11-MUA) for the first time. The principle for the assay depends on the following features: (1) Cu2+-triggered fluorescence quenching of AuNCs owing to the binding between Cu2+ and the carboxyl groups, (2) the cleavage of BSA into amino acids/peptide fragments catalyzed by trypsin, and (3) the stronger binding ability between amino acids/peptide fragments and Cu2+ than that between AuNCs and Cu2+, accompanied by the conversion from fluorescence off-state to on-state. Under the optimized conditions, the proposed sensing assay shows a good linear relationship from 0.01 to 2μg/mL and provides an inspiring detection limit of 0.004μg/mL, which possesses adequate sensitivity for practical samples of human urine. In addition, our proposed trypsin bioassay can be expanded into the inhibitor evaluation by using trypsin inhibitor from soybean as a model. Such unprecedented AuNCs-based fluorescence “turn-on” analytical proposal for assessing trypsin activity gives a new sight to develop several other potential bioassays or enzyme activity assays with the help of different kinds of substrates in the near future.