Bright NIR-II Photoluminescence in Rod-Shaped Icosahedral Gold Nanoclusters

A probe for NIR-II imaging and multimodal analysis of early Alzheimer's disease by targeting CTGF

To date, earlier diagnosis of Alzheimer's disease (AD) is still challenging. Recent studies revealed the elevated expression of connective tissue growth factor (CTGF) in AD brain is an upstream regulator of amyloid-beta (Aβ) plaque, thus CTGF could be an earlier diagnostic biomarker of AD than Aβ plaque. Herein, we develop a peptide-coated gold nanocluster that specifically targets CTGF with high affinity (KD ~ 21.9 nM). The probe can well penetrate the blood-brain-barrier (BBB) of APP/PS1 transgenic mice at early-stage (earlier than 3-month-old) in vivo, allowing non-invasive NIR-II imaging of CTGF when there is no appearance of Aβ plaque deposition. Notably, this probe can also be applied to measuring CTGF on postmortem brain sections by multimodal analysis, including fluorescence imaging, peroxidase-like chromogenic imaging, and ICP-MS quantitation, which enables distinguishment between the brains of AD patients and healthy people. This probe possesses great potential for precise diagnosis of earlier AD before Aβ plaque formation.

Photoluminescent Characterization of Metal Nanoclusters: Basic Parameters, Methods, and Applications

Metal nanoclusters (MNCs) can be synthesized with atomically precise structures and molecule formulae due to the rapid development of nanocluster science in recent decades. The ultrasmall size range (normally < 2 nm) endows MNCs with plenty of molecular-like properties, among which photoluminescent properties have aroused extensive attention. Tracing the research and development processes of luminescent nanoclusters, various photoluminescent analysis and characterization methods play a significant role in elucidating luminescent mechanism and analyzing luminescent properties. In this review, it is aimed to systematically summarize the normally used photoluminescent characterizations in MNCs including basic parameters and methods, such as excitation/emission wavelength, quantum yield, and lifetime. For each key parameter, first its definition and meaning is introduced and then the relevant characterization methods including measuring principles and the revelation of luminescent properties from the collected data are discussed. Then, it is discussed in details how to explore the luminescent mechanism of MNCs and construct NC-based applications based on the measured data. By means of these characterization strategies, the luminescent properties of MNCs and NC-based designs can be explained quantitatively and qualitatively. Hence, this review is expected to provide clear guidance for researchers to characterize luminescent MNCs and better understand the luminescent mechanism from the measured results.

Eight-Electron Copper Nanoclusters for Photothermal Conversion

Owing to distinct physicochemical properties in comparison to gold and silver counterparts, atomically precise copper nanoclusters are attracting embryonic interest in material science. The introduction of copper cluster nanomaterials in more interesting fields is currently urgent and desired. Reported in this work are novel copper nanoclusters of [XCu54Cl12(tBuS)20(NO3)12] (X=S or none, tBuSH=2-methyl-2-propanethiol), which exhibit high performance in photothermal conversion. The clusters have been prepared in one pot and characterized by combinatorial techniques including ultraviolet-visible spectroscopy (UV-vis), electrospray ionization mass spectrometry (ESI-MS), and X-ray photoelectron spectroscopy (XPS). The molecular structure of the clusters, as revealed by single crystal X-ray diffraction analysis (SCXRD), shows the concentric three-shell Russian doll arrangement of X@Cu14@Cl12@Cu40. Interestingly, the [SCu54Cl12(tBuS)20(NO3)12] cluster contains 8 free valence electrons in its structure, making it the first eight-electron copper nanocluster stabilized by thiolates. More impressively, the clusters possess an effective photothermal conversion (temperature increases by 71 °C within ~50 s, λex=445 nm, 0.5 W cm-2) in a wide wavelength range (either blue or near-infrared). The photothermal conversion can be even driven under irradiation of simulated sunlight (3 sun), endowing the clusters with great potency in solar energy utilization.

Dithiocarbonate-Protected Au25 Nanorods of a Chiral D5 Configuration and NIR-II Phosphorescence

Innovative surface-protecting ligands are in constant demand due to their crucial role in shaping the configuration, property, and application of gold nanoclusters. Here, the unprecedented O-ethyl dithiocarbonate (DTX)-stabilized atomically precise gold nanoclusters, [Au25(PPh3)10(DTX)5Cl2]2+ (Au25DTX-Cl) and [Au25(PPh3)10(DTX)5Br2]2+ (Au25DTX-Br), were synthesized and structurally characterized. The introduction of bidentate DTX ligands not only endowed the gold nanocluster with unique staggered Au25 nanorod configurations but also generated the symmetry breaking from the D5d geometry of the Au25 kernels to the chiral D5 configuration of the Au25 molecules. The chirality of Au25 nanorods was notably revealed through single-crystal X-ray diffraction, and chiral separation was induced by employing chiral DTX ligands. The staggered configurations of Au25 nanorods, as opposed to eclipsed ones, were responsible for the large red shift in the emission wavelengths, giving rise to a promising near-infrared II (NIR-II, >1000 nm) phosphorescence. Furthermore, their performances in photocatalytic sulfide oxidation and electrocatalytic hydrogen evolution reactions have been examined, and it has been demonstrated that the outstanding catalytic activity of gold nanoclusters is highly related to their stability.

Solvent-Mediated Hetero/Homo-Phase Crystallization of Copper Nanoclusters and Superatomic Kernel-Related NIR Phosphorescence

Atomically precise superatomic copper nanoclusters (Cu NCs) have been the subject of immense interest for their intriguing structures and diverse properties; nonetheless, the variable oxidation state of copper ions and complex solvation effects in wet synthesis systems pose significant challenges for comprehending their synthesis and crystallization mechanism. Herein, we present a solvent-mediated approach for the synthesis of two Cu NCs, namely, superatomic Cu26 and pure-Cu(I) Cu16. They initially formed as a hetero-phase and then separated as a homo-phase via modulating binary solvent composition. In situ UV/vis absorption and electrospray ionization mass spectra revealed that the solvent-mediated assembly was determined to be the underlying mechanism of hetero/homo-phase crystallization. Cu26 is a 2-electron superatom with a kernel-shell structure that includes a [Cu20Se12]4- shell and [Cu6]4+ kernel, containing two 1S jellium electrons. Conversely, Cu16 is a pure-Cu(I) Cu/Se nanocluster that features a [Cu16Se6]4+ core protected by extra dimercaptomaleonitrile ligands. Remarkably, Cu26 exhibits unique near-infrared phosphorescence (NIR PH) at 933 nm due to the presence of a superatomic kernel-related charge transfer state (3MM(Cu)CT). Overall, this work not only showcases the hetero/homo-phase crystallization of Cu NCs driven by a solvent-mediated assembly mechanism but also enables the rare occurrence of NIR PH within the 2-electron copper superatom family.

Building Block Metal Nanocluster-Based Growth in 1D Direction

Metal nanoclusters with precisely modulated structures at the nanoscale give us the opportunity to synthesize and investigate 1D nanomaterials at the atomic level. Herein, it realizes selective 1D growth of building block nanocluster "Au13 Cd2 " into three structurally different nanoclusters: "hand-in-hand" (Au13 Cd2 )2 O, "head-to-head" Au25 , and "shoulder-to-shoulder" Au33 . Detailed studies further reveals the growth mechanism and the growth-related tunable properties. This work provides new hints for the predictable structural transformation of nanoclusters and atomically precise construction of 1D nanomaterials.

Visible to NIR-II Photoluminescence of Atomically Precise Gold Nanoclusters

Atomically precise gold nanoclusters (NCs) have emerged as a new class of precision materials and attracted wide interest in recent years. One of the unique properties of such nanoclusters pertains to their photoluminescence (PL), for it can widely span visible to near-infrared-I and -II wavelengths (NIR-I/II), and even beyond 1700 nm by manipulating the size, structure, and composition. The current research efforts focus on the structure-PL correlation and the development of strategies for raising the PL quantum yields, which is nontrivial when moving from the visible to the near-infrared wavelengths, especially in the NIR-II regions. This review summarizes the recent progress in the field, including i) the types of PL observed in gold NCs such as fluorescence, phosphorescence, and thermally activated delayed fluorescence, as well as dual emission; ii) some effective strategies that are devised to improve the PL quantum yield (QY) of gold NCs, such as heterometal doping, surface rigidification, and core phonon engineering, with double-digit QYs for the NIR PL on the horizons; and iii) the applications of luminescent gold NCs in bioimaging, photosensitization, and optoelectronics. Finally, the remaining challenges and opportunities for future research are highlighted.

Intratumor injected gold molecular clusters for NIR-II imaging and cancer therapy

Surgical resections of solid tumors guided by visual inspection of tumor margins have been performed for over a century to treat cancer. Near-infrared (NIR) fluorescence labeling/imaging of tumor in the NIR-I (800 to 900 nm) range with systemically administrated fluorophore/tumor-targeting antibody conjugates have been introduced to improve tumor margin delineation, tumor removal accuracy, and patient survival. Here, we show Au25 molecular clusters functionalized with phosphorylcholine ligands (AuPC, ~2 nm in size) as a preclinical intratumorally injectable agent for NIR-II/SWIR (1,000 to 3,000 nm) fluorescence imaging-guided tumor resection. The AuPC clusters were found to be uniformly distributed in the 4T1 murine breast cancer tumor upon intratumor (i.t.) injection. The phosphocholine coating afforded highly stealth clusters, allowing a high percentage of AuPC to fill the tumor interstitial fluid space homogeneously. Intra-operative surgical navigation guided by imaging of the NIR-II fluorescence of AuPC allowed for complete and non-excessive tumor resection. The AuPC in tumors were also employed as a photothermal therapy (PTT) agent to uniformly heat up and eradicate tumors. Further, we performed in vivo NIR-IIb (1,500 to 1,700 nm) molecular imaging of the treated tumor using a quantum dot-Annexin V (QD-P3-Anx V) conjugate, revealing cancer cell apoptosis following PTT. The therapeutic functionalities of AuPC clusters combined with rapid renal excretion, high biocompatibility, and safety make them promising for clinical translation.

[Au14(2-SAdm)9(Dppe)2]+: a gold nanocluster with a crystallization-induced emission enhancement phenomenon

In this work, a gold nanocluster [Au14(2-SAdm)9(Dppe)2]+ was synthesized and structurally determined by X-ray crystallography. The crystals of this cluster exhibit a 50-fold enhancement in quantum yield (5.05% for crystals) compared with its solution. Crystallographic analysis reveals that the weak intermolecular interactions (C-H⋯π, π⋯π) can inhibit the molecular vibration and thus generate the crystallization-induced emission enhancement phenomenon.

Imaging-Guided Antibacterial Based on Gold Nanocrystals and Assemblies

Bacterial infection becomes a severe threat to human life and health worldwide. Antibiotics with the ability to resist pathogenic bacteria are therefore widely used, but the misuse or abuse of antibiotics can generate multidrug-resistant bacteria or resistant biofilms. Advanced antibacterial technologies are needed to counter the rapid emergence of drug-resistant bacteria. With the excellent optical properties, engineerable surface chemistry, neglectable biotoxicity, gold nanocrystals are particularly attractive in biomedicine for cancer therapy and antibacterial therapy, as well as nanoprobes for bioimaging and disease diagnosis. In this perspective, gold nanocrystal-based antibacterial performance and deep-tissue imaging are summarized, including near-infrared-light excited photoacoustic imaging and fluorescence imaging through deep tissue infections. On the basis of integrating "imaging-therapy-targeting" in single nanotheranostic, the current challenges of imaging-guided antibacterial and therapy based on gold nanocrystals are discussed, and some insights are provided into the gold nanocrystal-based nanoplatform that integrates antibacterial activity and therapy. This perspective is expected to provide comprehensive guidance for diagnosing and combating bacterial infections based on gold nanostructures.

Molecularly Engineered Room-Temperature Phosphorescence for Biomedical Application: From the Visible toward Second Near-Infrared Window

Phosphorescence, characterized by luminescent lifetimes significantly longer than that of biological autofluorescence under ambient environment, is of great value for biomedical applications. Academic evidence of fluorescence imaging indicates that virtually all imaging metrics (sensitivity, resolution, and penetration depths) are improved when progressing into longer wavelength regions, especially the recently reported second near-infrared (NIR-II, 1000-1700 nm) window. Although the emission wavelength of probes does matter, it is not clear whether the guideline of "the longer the wavelength, the better the imaging effect" is still suitable for developing phosphorescent probes. For tissue-specific bioimaging, long-lived probes, even if they emit visible phosphorescence, enable accurate visualization of large deep tissues. For studies dealing with bioimaging of tiny biological architectures or dynamic physiopathological activities, the prerequisite is rigorous planning of long-wavelength phosphorescence, being aware of the cooperative contribution of long wavelengths and long lifetimes for improving the spatiotemporal resolution, penetration depth, and sensitivity of bioimaging. In this Review, emerging molecular engineering methods of room-temperature phosphorescence are discussed through the lens of photophysical mechanisms. We highlight the roles of phosphorescence with emission from visible to NIR-II windows toward bioapplications. To appreciate such advances, challenges and prospects in rapidly growing studies of room-temperature phosphorescence are described.

pH-Responsive Near-Infrared Emitting Gold Nanoclusters

Near-infrared (NIR) fluorophores with pH-responsive properties suggest merits in biological analyses. This work establishes a general and effective method to obtain pH-responsive NIR emissive gold nanoclusters by introducing aliphatic tertiary amine (TA) groups into the ligands. Computational study suggests that the pH-responsive NIR emission is associated with electronic structure change upon protonation and deprotonation of TA groups. Photo-induced electron transfer between deprotonated TA groups and the surface Au-S motifs of gold nanoclusters can disrupt the radiative transitions and thereby decrease the photoluminescence intensity in basic environments (pH=7-11). By contrast, protonated TA groups curb the electron transfer and restore the photoluminescence intensity in acidic environments (pH=4-7). The pH-responsive NIR-emitting gold nanoclusters serve as a specific and sensitive probe for the lysosomes in the cells, offering non-invasive emissions without interferences from intracellular autofluorescence.

Core-Packing-Related Vibrational Properties of Thiol-Protected Gold Nanoclusters and Their Excited-State Behavior

Thiolate-protected gold nanoclusters, with unique nuclearity- and structure-dependent properties, have been extensively used in energy conversion and catalysis; however, the mystery between kernel structures and properties remains to be revealed. Here, the influence of core packing on the electronic structure, vibrational properties, and excited-state dynamics of four gold nanoclusters with various kernel structures is explored using density functional theory combined with time-domain nonadiabatic molecular dynamics simulations. We elucidate the correlation between the geometrical structure and excited-state dynamics of gold nanoclusters. The distinct carrier lifetimes of the four nanoclusters are attributed to various electron-phonon couplings arising from the different vibrational properties caused by core packing. We have identified specific phonon modes that participate in the electron-hole recombination dynamics, which are related to the gold core of nanoclusters. This study paints a physical picture from the geometric configuration, electronic structure, vibrational properties, and carrier lifetime of these nanoclusters, thereby facilitating their potential application in optoelectronic materials.

Near-Infrared Dual-Emission of a Thiolate-Protected Au42 Nanocluster: Excited States, Nonradiative Rates, and Mechanism

Both DFT and TD-DFT methods are used to elaborate on the excited-state properties and dual-emission mechanism of a thiolate-protected Au42 nanocluster. A three-state model (S0, S1, and T1) is proposed with respect to the results. The intersystem crossing (ISC) process from S1 to T1 benefits from a small reorganization energy due to the similar geometric structures of S1 and T1. However, the ISC process is suppressed by relatively small spin-orbit coupling resulting from the similarity of the electronic structures of S1 and T1. As a result of the counterbalance, the ISC rate is comparable with the fluorescence emission rate. In the T1 state, the phosphorescence emission prevails the reverse ISC process back to the S1 state. Taken together, fluorescence and phosphorescence are achieved simultaneously. The present work provides deep mechanistic insights to aid the rational design of NIR dual-emissive metal nanoclusters.

Evolution of Electrocatalytic CO2 Reduction Activity Induced by Charge Segregation in Atomically Precise AuAg Nanoclusters Based on Icosahedral M13 Unit 3D Assembly

The precise self-assembly of building blocks at atomic level provides the opportunity to achieve clusters with advanced catalytic properties. However, most of the current self-assembled materials are fabricated by 1/2D assembly of blocks. High dimensional (that is, 3D) assembly is widely believed to improve the performance of cluster. Herein, the effect of 3D assembly on the activity for electrocatalytic CO2 reduction reaction (CO2 RR) is investigated by using a range of clusters (Au8 Ag55 , Au8 Ag57 , Au12 Ag60 ) based on 3D assembly of M13 unit as models. Although three clusters have almost the same sizes and geometric structures, Au8 Ag55 exhibits the best CO2 RR performance due to the strong CO2 adsorption capacity and effective inhibition of H2 evolution competition reaction. The deep insight into the superior activity of Au8 Ag55 is the unique electronic structure attributed to the charge segregation. This study not only demonstrates that the assembly mode greatly affects the catalytic activity, but also offers an idea for rational designing and precisely constructing catalysts with controllable activities.

Tracking tumor heterogeneity and progression with near-infrared II fluorophores

Heterogeneous cells are the main feature of tumors with unique genetic and phenotypic characteristics, which can stimulate differentially the progression, metastasis, and drug resistance. Importantly, heterogeneity is pervasive in human malignant tumors, and identification of the degree of tumor heterogeneity in individual tumors and progression is a critical task for tumor treatment. However, current medical tests cannot meet these needs; in particular, the need for noninvasive visualization of single-cell heterogeneity. Near-infrared II (NIR-II, 1000-1700 nm) imaging exhibits an exciting prospect for non-invasive monitoring due to the high temporal-spatial resolution. More importantly, NIR-II imaging displays more extended tissue penetration depths and reduced tissue backgrounds because of the significantly lower photon scattering and tissue autofluorescence than traditional the near-infrared I (NIR-I) imaging. In this review, we summarize systematically the advances made in NIR-II in tumor imaging, especially in the detection of tumor heterogeneity and progression as well as in tumor treatment. As a non-invasive visual inspection modality, NIR-II imaging shows promising prospects for understanding the differences in tumor heterogeneity and progression and is envisioned to have the potential to be used clinically.

Second Near-Infrared (NIR-II) Window for Imaging-Navigated Modulation of Brain Structure and Function

For a long time, optical imaging of the deep brain with high resolution has been a challenge. Recently, with the advance in second near-infrared (NIR-II) bioimaging techniques and imaging contrast agents, NIR-II window bioimaging has attracted great attention to monitoring deeper biological or pathophysiological processes with high signal-to-noise ratio (SNR) and spatiotemporal resolution. Assisted with NIR-II bioimaging, the modulation of structure and function of brain is promising to be noninvasive and more precise. Herein, in this review, first the advantage of NIR-II light in brain imaging from the interaction between NIR-II and tissue is elaborated. Then, several specific NIR-II bioimaging technologies are introduced, including NIR-II fluorescence imaging, multiphoton fluorescence imaging, and photoacoustic imaging. Furthermore, the corresponding contrast agents are summarized. Next, the application of various NIR-II bioimaging technologies in visualizing the characteristics of cerebrovascular network and monitoring the changes of the pathology signals will be presented. After that, the modulation of brain structure and function based on NIR-II bioimaging will be discussed, including treatment of glioblastoma, guidance of cell transplantation, and neuromodulation. In the end, future perspectives that would help improve the clinical translation of NIR-II light are proposed.

Two triplet emitting states in one emitter: Near-infrared dual-phosphorescent Au20 nanocluster

Intrinsic dual-emission (DE) of gold nanoclusters in the near-infrared (NIR) are fascinating for fundamental importance and practical applications, but their synthesis remains a formidable challenge and sophisticated excited-state processes make elucidating DE mechanisms much more arduous. Here, we report an all-alkynyl-protected gold nanocluster, Au20, showing a prolate Au₁₂ tri-octahedral kernel surrounded by two Au₂(CZ-PrA)₃ dimers, four Au(CZ-PrA)₂ monomers, and two CZ-PrA^- bridges. Au20 exhibits distinguished photophysical properties including NIR DE at 820 and 940 nm, microsecond radiative relaxation, and 6.26% photoluminescent quantum yield at ambient environment in nondegassed solution. Combining systematic studies on steady/transient spectroscopy and theoretical calculation, we identified two triplet charge transfer (CT) states, ligand-to-kernel and kernel-based CT states as DE origins. Furthermore, this NIR DE exhibits highly independent and sensitive response to surrounding environments, which well coincide with its mechanism. This work not only provides a substantial structure model to understand a distinctive DE mechanism but also motivates the further development of NIR DE materials.

Emerging ultrasmall luminescent nanoprobes for in vivo bioimaging

Photoluminescence (PL) imaging has become a fundamental tool in disease diagnosis, therapeutic evaluation, and surgical navigation applications. However, it remains a big challenge to engineer nanoprobes for high-efficiency in vivo imaging and clinical translation. Recent years have witnessed increasing research efforts devoted into engineering sub-10 nm ultrasmall nanoprobes for in vivo PL imaging, which offer the advantages of efficient body clearance, desired clinical translation potential, and high imaging signal-to-noise ratio. In this review, we present a comprehensive summary and contrastive discussion of emerging ultrasmall luminescent nanoprobes towards in vivo PL bioimaging of diseases. We first summarize size-dependent nano-bio interactions and imaging features, illustrating the unique attributes and advantages/disadvantages of ultrasmall nanoprobes differentiating them from molecular and large-sized probes. We also discuss general design methodologies and PL properties of emerging ultrasmall luminescent nanoprobes, which are established based on quantum dots, metal nanoclusters, lanthanide-doped nanoparticles, and silicon nanoparticles. Then, recent advances of ultrasmall luminescent nanoprobes are highlighted by surveying their latest in vivo PL imaging applications. Finally, we discuss existing challenges in this exciting field and propose some strategies to improve in vivo PL bioimaging and further propel their clinical applications.

Overall structure of Au12Ag60(S-c-C6H11)31Br9(Dppp)6: achieving a stronger assembly of icosahedral M13 units

Precise atomically assembled nanoclusters provide a great platform to elucidate the evolution of the assembly of building blocks. Herein, a large icosahedral (M₁₃)-based silver-gold alloy nanocluster [Au₁₂Ag₆₀(S-c-C₆H₁₁)₃₁Br₉(Dppp)₆]Br₂ (dppp = 1,3-bis(diphenylphosphino)propane) is reported. Structurally, Au₁₂Ag₆₀ consists of an Au₁₂Ag₄₀ kernel, which is viewed as the interpenetration of ten twisted complete icosahedrons (M₁₃) and two missing icosahedrons (M₁₂), and this is surrounded by a complex metal-organic shell. Benefiting from the extra doping of eight to twelve Au atoms, the octameric assembly was increased to a twelve-mer assembly. The time-dependent density functional theory (TDDFT) method with a Tamm-Dancoff approximation (TDA) was performed to investigate the difference in the optical properties of Au₁₂Ag₆₀ and Au₈Ag₅₇. The results indicate that the difference in the amount of Au atoms results in different optical properties. Furthermore, transient absorption spectroscopy (TA) was also performed, revealing that a twelve-mer assembly greatly enhances the excited-state lifetime. The [Au₁₂Ag₆₀(S-c-C₆H₁₁)₃₁Br₉(Dppp)₆]Br₂ alloy nanocluster has provided a breakthrough in the number of icosahedral M₁₃ assemblies, i.e., achieving a twelve-mer assembly, helping to elucidate the fusion growth of M₁₃-based assembled nanoclusters as well as their geometric/electronic structure correlations, which will promote further research on the assembly of M₁₃ nano-building blocks, especially on their optical properties.

Direct Observation of the Pressure-Induced Structural Variation in Gold Nanoclusters and the Correlated Optical Response

The ability to gradually modify the atomic structures of nanomaterials and directly identify such structural variation is important in nanoscience research. Here, we present the first example of a high-pressure single-crystal X-ray diffraction analysis of atomically precise metal nanoclusters. The pressure-dependent, subangstrom structural evolution of an ultrasmall gold nanoparticle, Au₂₅S₁₈, has been directly identified. We found that a 0.1 Å decrease of the Au-Au bond length could induce a blue-shift of 30 nm in the photoluminescence spectra of gold nanoclusters. From theoretical calculations, the origins of the blue-shift and enhanced photoluminescence under pressure are investigated, which are ascribed to molecular orbital symmetry and conformational locking, respectively. The combination of the high-pressure in situ X-ray results with both theoretical and experimental optical spectra provides a direct and generalizable avenue to unveil the underlying structure-property relations for nanoclusters and nanoparticles which cannot be obtained through traditional physical chemistry measurements.

Tailoring the Electron-Phonon Interaction in Au25(SR)18 Nanoclusters via Ligand Engineering and Insight into Luminescence

Understanding the electron-phonon interaction in Au nanoclusters (NCs) is essential for enhancing and tuning their photoluminescence (PL) properties. Among all the methods, ligand engineering is the most straightforward and facile one to design Au NCs with the desired PL properties. However, a systematic understanding of the ligand effects toward electron-phonon interactions in Au NCs is still missing. Herein, we synthesized four Au₂₅(SR)₁₈^- NCs protected by different -SR ligands and carefully examined their temperature-dependent band-gap renormalization behavior. Data analysis by a Bose-Einstein two-oscillator model revealed a suppression of high-frequency optical phonons in aromatic-ligand-protected Au₂₅ NCs. Meanwhile, a low-frequency breathing mode and a quadrupolar mode are attributed as the main contributors to the phonon-assisted nonradiative relaxation pathway in aromatic-ligand-protected Au₂₅ NCs, which is in contrast with non-aromatic-ligand-protected Au₂₅ NCs, in which tangential and radial modes play the key roles. The PL measurements of the four Au₂₅ NCs showed that the suppression of optical phonons led to higher quantum yields in aromatic-ligand-protected Au₂₅ NCs. Cryogenic PL measurements provide insights into the nonradiative energy relaxation, which should be further investigated for a full understanding of the PL mechanism in Au₂₅ NCs.

Mechanical nanolattices printed using nanocluster-based photoresists

Natural materials exhibit emergent mechanical properties as a result of their nanoarchitected, nanocomposite structures with optimized hierarchy, anisotropy, and nanoporosity. Fabrication of such complex systems is currently challenging because high-quality three-dimensional (3D) nanoprinting is mostly limited to simple, homogeneous materials. We report a strategy for the rapid nanoprinting of complex structural nanocomposites using metal nanoclusters. These ultrasmall, quantum-confined nanoclusters function as highly sensitive two-photon activators and simultaneously serve as precursors for mechanical reinforcements and nanoscale porogens. Nanocomposites with complex 3D architectures are printed, as well as structures with tunable, hierarchical, and anisotropic nanoporosity. Nanocluster-polymer nanolattices exhibit high specific strength, energy absorption, deformability, and recoverability. This framework provides a generalizable, versatile approach for the use of photoactive nanomaterials in additive manufacturing of complex systems with emergent mechanical properties.

Luminescent Gold Nanoparticles with Controllable Hydrophobic Interactions

The construction of luminescent gold nanoparticles (AuNPs) with highly redshifted emission in the second near-infrared window (NIR-II) and good biocompatibility is still challenging. Herein, using an amphiphilic block copolymer (ABC) template with controllable hydrophobic interactions in the diverse forms of unimers and micelles, we report a facile strategy for redshifting the emission and enhancing the biological interactions of luminescent AuNPs. While the uniform clusters of NIR-II AuNPs are formed in situ inside the hydrophobic cores of ABC micelles with strong interparticle hydrophobic interactions and enhanced emission at 1080 nm with a high quantum yield (QY) of 1.6%, the rigid NIR-II AuNPs are generated with strong intraparticle hydrophobic interactions as ABC unimers on the surface, leading to a redshifted emission of 1280 nm with a QY of 0.25% and enhancing the affinities toward injured intestinal mucosa in colitis imaging. These findings open new possibilities for the design of highly redshifted luminescent AuNPs with enhanced biological interactions.

Near-Infrared Dual Emission from the Au42(SR)32 Nanocluster and Tailoring of Intersystem Crossing

This work presents the synthesis and intriguing photoluminescence of the Au₄₂(PET)₃₂ (PET = 2-phenylethanethiolate) nanocluster (NC). The Au₄₂(PET)₃₂ NC exhibits dual emission at 875 and 1040 nm, which are revealed to be fluorescence and phosphorescence, respectively. The emission quantum yield (QY) of Au₄₂(PET)₃₂ in dichloromethane is 11.9% at room temperature in air, which is quite rare for thiolate-protected Au NCs. When Au₄₂(PET)₃₂ NCs are embedded in polystyrene films (solid state), the fluorescence was dramatically suppressed while the phosphorescence was significantly enhanced. This divergent behavior is explained by dipolar interaction-induced enhancement of intersystem crossing from singlet to triplet excited state.

Phosphorylcholine-conjugated gold-molecular clusters improve signal for Lymph Node NIR-II fluorescence imaging in preclinical cancer models

Sentinel lymph node imaging and biopsy is important to clinical assessment of cancer metastasis, and novel non-radioactive lymphographic tracers have been actively pursued over the years. Here, we develop gold molecular clusters (Au₂₅) functionalized by phosphorylcholine (PC) ligands for NIR-II (1000–3000 nm) fluorescence imaging of draining lymph nodes in 4T1 murine breast cancer and CT26 colon cancer tumor mouse models. The Au-phosphorylcholine (Au-PC) probes exhibit ‘super-stealth’ behavior with little interactions with serum proteins, cells and tissues in vivo, which differs from the indocyanine green (ICG) dye. Subcutaneous injection of Au-PC allows lymph node mapping by NIR-II fluorescence imaging at an optimal time of ~ 0.5 − 1 hour postinjection followed by rapid renal clearance. Preclinical NIR-II fluorescence LN imaging with Au-PC affords high signal to background ratios and high safety and biocompatibility, promising for future clinical translation.

Fluorescent tracers facilitate the identification and subsequent collection of tumour draining lymph node biopsies, enabling important clinical assessment. Here, the authors present a molecular gold nanocluster NIR-II fluorescent imaging probe and demonstrate its utility to visualise draining lymph nodes in breast and colon cancer mouse models.

Lethal Interactions of Atomically Precise Gold Nanoclusters and Pseudomonas aeruginosa and Staphylococcus aureus Bacterial Cells

Ultrasmall metal nanoclusters (NCs) are employed in an array of diagnostic and therapeutic applications due to their tunable photoluminescence, high biocompatibility, polyvalent effect, ease of modification, and photothermal stability. However, gold nanoclusters' (AuNCs') intrinsically antimicrobial properties remain poorly explored and are not well understood. Here, we share an insight into the antimicrobial action of atomically precise AuNCs based on their ability to passively translocate across the bacterial membrane. Functionalized by a hydrophilic modified-bidentate sulfobetaine zwitterionic molecule (AuNC-ZwBuEt) or a more hydrophobic monodentate-thiolate, mercaptohexanoic acid (AuNC-MHA) molecule, 2 nm AuNCs were lethal to both Gram-negative Pseudomonas aeruginosa and Gram-positive Staphylococcus aureus bacteria. The bactericidal efficiency was found to be bacterial strain-, time-, and concentration-dependent. The direct visualizations of the translocation of AuNCs and AuNC-cell and subcellular interactions were investigated using cryo-soft X-ray nano-tomography, transmission electron microscopy (TEM), and scanning TEM energy-dispersive spectroscopy analyses. AuNC-MHA were identified in the bacterial cytoplasm within 30 min, without evidence of the loss of membrane integrity. It is proposed that the bactericidal effect of AuNCs is attributed to their size, which allows for efficient energy-independent translocation across the cell membrane. The internalization of both AuNCs caused massive internal damage to the cells, including collapsed subcellular structures and altered cell morphology, leading to the eventual loss of cellular integrity.

New Xanthene Dyes with NIR-II Emission Beyond 1200 nm for Efficient Tumor Angiography and Photothermal Therapy

Fluorescence (FL) bioimaging in the second near-infrared window (NIR-II, 1000-1700 nm) provides improved imaging quality and high resolution for diagnosis of deep-seated tumors. However, integrating FL bioimaging and photothermal therapy (PTT) in a single photoactive molecule exhibits a great challenge because a dramatic increase of PTT in the NIR-II window benefitting from the nonradiative decay will sacrifice the fluorescence brightness that is unfavorable for FL bioimaging. Therefore, balancing the radiative decay and nonradiative decay is an effective and rational design strategy. Herein, four NIR-II xanthene dyes (CL1-CL4) are synthesized with maximal emission beyond 1200 nm under 1064 nm excitation. CL4 exhibits the largest fluorescence quantum yield and a significant fluorescence enhancement after complexation with fetal bovine serum (FBS). As-prepared CL4/FBS has a maximal emission of 1235 nm and a high photothermal conversion efficiency of 36% under 1064 nm excitation. Bright and refined tumor vessels with a fine resolution of 0.23 mm can be clearly distinguished by CL4/FBS. In vivo studies show that a balanced utilization of fluorescence and photothermy in the NIR-II window is successfully achieved with superior biocompatibility. This efficient strategy provides promising avenue for precise theranostics of deep tumors.

Evidence for triplet-state-dominated luminescence in biicosahedral superatomic molecular Au25 clusters

In photoluminescence (PL) quenching and triplet fusion upconversion experiments with fluorescent organic-molecule quenchers, it was revealed that a rod-shaped, phosphine- and thiolate-protected biicosahedral Au₂₅ cluster (a representative di-superatomic molecule) exhibits only phosphorescence, not fluorescence, at room temperature with an intersystem crossing quantum yield of almost 100%. By virtue of these photophysical properties, this cluster can be used as a triplet sensitizer that undergoes direct singlet-triplet transitions in the near-infrared (NIR) region (730-900 nm), inducing photon upconversion from NIR to visible light.

A persistent luminescence resonance energy transfer-based molecular beacon probe for the highly sensitive detection of microRNA in biological samples

Herein, a time-resolved luminescence resonance energy transfer (TR-LRET) molecular beacon (MB) probe employing persistent luminescence nanoparticles (PLNPs) as the energy donors was first constructed, and further designed for microRNA21 (miR21) sensing. This probe (named as PLNPs-MB) was facilely fabricated by covalent bioconjugation between poly-(acrylic acid) (PAA) modified near-infrared (NIR) emissive PLNPs i.e. ZnGa₂O₄:Cr³⁺ and functionalized MB oligonucleotide (5'-NH₂ and 3'-BHQ3). Accordingly, PLNPs and BHQ3 were in close proximity to each other, leading to the occurrence of LRET and obvious persistent luminescence (PL) quenching. In the presence of miR21, loop of the PLNP-MB was hybridized, accompanying BHQ3 away from PLNPs and the restraint of LRET process. As a result, PL of the PLNPs was recovered, which built the foundation of miR21 quantification. The probe provided a linear response range from 0.1 to 10 nM for miR21 detection. Quantification limit of this probe was competitive and about 1-2 orders of magnitude lower than that of other reported MB probes for nucleic acid. Moreover, the proposed probe was successfully adopted for miR21 detection in biological fluids (human serum, cell extraction). This work also provided a sensitive detection nanoplatform for other targets through modifying diverse MBs onto the surface of PLNPs.

Tailoring the SWIR emission of gold nanoclusters by surface ligand rigidification and their application in 3D bioimaging

The influence of solvent polarity and surface ligand rigidification on the SWIR emission profile of gold nanoclusters with an anistropic surface was investigated. A strong enhancement of the SWIR emission band at 1200 nm was observed when measuring in different local environments: in solution, in polymer composites, and in solids. SWIR in vivo imaging of mice assisted by deep learning after intravenous administration of these gold nanoclusters provides high definition pseudo-3D views of vascular blood vessels.

Effect of single electrons on the excited state dynamics of rod-shaped Au25 nanoclusters

The excited state dynamics of small-sized metal nanoclusters are dependent on their crystal structures, while the effect of the charge state remains largely unknown. Here, we report the influence of single electrons on the excited-state dynamics of non-superatom Au clusters by comparing the transient absorption isotropy and anisotropy dynamics of two rod-shaped Au₂₅ nanoclusters protected by organic ligands. Two decay lifetimes (0.9 ps and 2.3 μs) can be identified in the excited state relaxation of Au₂₅²⁺ rods, which are assigned to the internal conversion from a higher to lower excited state and the relaxation to the ground state, respectively. With the addition of one electron, an additional 660 ps decay is observed in Au₂₅⁺, which should originate from the presence of a single electron occupied molecular orbital. Transient anisotropy measurements reveal a 500 ps rotational diffusion process in both the nanoclusters, while the initial dipole moment orientation is found to be highly dependent on the charge state. These results are of importance to understanding the effect of the charge state on the optical properties of metal nanoclusters.

Near-Infrared Inorganic Nanomaterials for Precise Diagnosis and Therapy

Traditional wavelengths (400–700 nm) have made tremendous inroads in vivo fluorescence imaging. However, the ability of visible light photon penetration hampered the bio-applications. With reduced photon scattering, minimal tissue absorption and negligible autofluorescence properties, near-infrared light (NIR 700–1700 nm) demonstrates better resolution, high signal-to-background ratios, and deep tissue penetration capability, which will be of great significance for in-vivo determination in deep tissue. In this review, we summarized the latest novel NIR inorganic nanomaterials and the emission mechanism including single-walled carbon nanotubes, rare-earth nanoparticles, quantum dots, metal nanomaterials. Subsequently, the recent progress of precise noninvasive diagnosis in biomedicine and cancer therapy utilizing near-infrared inorganic nanomaterials are discussed. In addition, this review will highlight the concerns, challenges and future directions of near-infrared light utilization.

Source of Bright Near-Infrared Luminescence in Gold Nanoclusters

Gold nanoclusters with near-infrared (NIR) photoluminescence (PL) have great potential as sensing and imaging materials in biomedical and bioimaging applications. In this work, Au₂₁(S-Adm)₁₅ and Au₃₈S₂(S-Adm)₂₀ are used to unravel the underlying mechanisms for the improved quantum yields (QY), large Stokes shifts, and long PL lifetimes in gold nanoclusters. Both nanoclusters show decent PL QY. In particular, the Au₃₈S₂(S-Adm)₂₀ nanocluster shows a bright NIR PL at 900 nm with QY up to 15% in normal solvents (such as toluene) at ambient conditions. The relatively lower QY for Au₂₁(S-Adm)₁₅ (4%) compared to that of Au₃₈S₂(S-Adm)₂₀ is attributed to the lowest-lying excited state being symmetry-disallowed, as evidenced by the pressure-dependent antispectral shift of the absorption spectra compared to PL, yet Au₂₁(S-Adm)₁₅ maintains some emissive properties due to a nearby symmetry-allowed excited state. Furthermore, our results show that suppression of nonradiative decay due to the surface "lock rings", which encircle the Au kernel and the surface "lock atoms" which bridge the fundamental Au kernel units (e.g., tetrahedra, icosahedra, etc.), is the key to obtaining high QYs in gold nanoclusters. The complicated excited-state processes and the small absorption coefficient of the band-edge transition lead to the large Stokes shifts and the long PL lifetimes that are widely observed in gold nanoclusters.

Versatile Types of Inorganic/Organic NIR-IIa/IIb Fluorophores: From Strategic Design toward Molecular Imaging and Theranostics

In vivo imaging in the second near-infrared window (NIR-II, 1000-1700 nm), which enables us to look deeply into living subjects, is producing marvelous opportunities for biomedical research and clinical applications. Very recently, there has been an upsurge of interdisciplinary studies focusing on developing versatile types of inorganic/organic fluorophores that can be used for noninvasive NIR-IIa/IIb imaging (NIR-IIa, 1300-1400 nm; NIR-IIb, 1500-1700 nm) with near-zero tissue autofluorescence and deeper tissue penetration. This review provides an overview of the reports published to date on the design, properties, molecular imaging, and theranostics of inorganic/organic NIR-IIa/IIb fluorophores. First, we summarize the design concepts of the up-to-date functional NIR-IIa/IIb biomaterials, in the order of single-walled carbon nanotubes (SWCNTs), quantum dots (QDs), rare-earth-doped nanoparticles (RENPs), and organic fluorophores (OFs). Then, these novel imaging modalities and versatile biomedical applications brought by these superior fluorescent properties are reviewed. Finally, challenges and perspectives for future clinical translation, aiming at boosting the clinical application progress of NIR-IIa and NIR-IIb imaging technology are highlighted.

Fluorescent carbon-dots enhance light harvesting and photosynthesis by overexpressing PsbP and PsiK genes

BACKGROUND

Fluorescent carbon-dots (CDs) with multifaceted advantages have provided hope for improvement of crop growth. Near infrared (NIR) CDs would be more competitive and promising than short-wavelength emissive CDs, which are not directly utilized by chloroplast. The molecular targets and underlying mechanism of these stimulative effects are rarely mentioned.

RESULTS

NIR-CDs with good mono-dispersity and hydrophily were easily prepared by a one-step microwave-assisted carbonization manner, which showed obvious UV absorptive and far-red emissive properties. The chloroplast-CDs complexes could accelerate the electron transfer from photosystem II (PS II) to photosystem I (PS I). NIR-CDs exhibited a concentration-dependent promotion effect on N. benthamiana growth by strengthening photosynthesis. We firstly demonstrated that potential mechanisms behind the photosynthesis-stimulating activity might be related to up-regulated expression of the photosynthesis and chloroplast synthesis related genes, among which PsbP and PsiK genes are the key regulators.

CONCLUSION

These results illustrated that NIR-CDs showed great potential in the applications to increase crop yields through ultraviolet light harvesting and elevated photosynthesis efficiency. This work would provide a theoretical basis for the understanding and applications of the luminescent nanomaterials (not limited to CDs) in the sunlight conversion-related sustainable agriculture.

Size-Scalable Near-Infrared Photoluminescence in Gold Monolayer Protected Clusters

Near-infrared photoluminescence of a series of three gold monolayer protected clusters (MPCs) with volumes spanning 50-200 ų was studied by using variable-temperature photoluminescence (VT-PL) spectroscopy. The three MPCs, which included Au₂₀(SC₈H₉)₁₅-diglyme, Au₂₅(SC₈H₉)₁₈, and Au₃₈(SC₁₂H₂₅)₂₄, all exhibited temperature-dependent intensities that reflected a few-millielectronvolt energy gap that separated bright emissive and dark nonradiative electronic states. All clusters showed increased PL intensities upon raising the sample temperature from 4.5 K to a cluster-specific value, upon which increased sample temperature resulted in emission quenching. The increased PL in the low-temperature range is attributed to thermally activated carrier transfer from dark to bright states. The quenching at elevated temperatures is attributed to nonradiative vibrational relaxation through Au-Au stretching of the MPCs metal core. Importantly, the results show evidence of a common and size scalable metal-centered intraband PL mechanism that is general for ultrasmall metal nanoclusters, which are expected to show nonscalable optical properties.