Ultrasensitive detection of toxic cations through changes in the tunnelling current across films of striped nanoparticles

The entropy-controlled strategy in self-assembling systems

Self-assembly of various building blocks has been considered as a powerful approach to generate novel materials with tailorable structures and optimal properties. Understanding physicochemical interactions and mechanisms related to structural formation and transitions is of essential importance for this approach. Although it is well-known that diverse forces and energies can significantly contribute to the structures and properties of self-assembling systems, the potential entropic contribution remains less well understood. The past few years have witnessed rapid progress in addressing the entropic effects on the structures, responses, and functions in the self-assembling systems, and many breakthroughs have been achieved. This review provides a framework regarding the entropy-controlled strategy of self-assembly, through which the structures and properties can be tailored by effectively tuning the entropic contribution and its interplay with the enthalpic counterpart. First, we focus on the fundamentals of entropy in thermodynamics and the entropy types that can be explored for self-assembly. Second, we discuss the rules of entropy in regulating the structural organization in self-assembly and delineate the entropic force and superentropic effect. Third, we introduce the basic principles, significance and approaches of the entropy-controlled strategy in self-assembly. Finally, we present the applications where this strategy has been employed in fields like colloids, macromolecular systems and nonequilibrium assembly. This review concludes with a discussion on future directions and future research opportunities for developing and applying the entropy-controlled strategy in complex self-assembling systems.

Amplitude-Resolved Single Particle Spectrophotometry: A Robust Tool for High-Throughput Size Characterization of Plasmonic Nanoparticles

Plasmonic nanoparticles have a wide range of applications in science and industry. Despite the numerous synthesis methods reported in the literature over the last decades, achieving precise control over the size and shape of large nanoparticle populations remains a challenge. Since variations in size and shape significantly affect the plasmonic properties of nanoparticles, accurate metrological techniques to characterize their morphological features are essential. Here, we present a novel spectrophotometric method, called Amplitude-Resolved Single Particle Spectrophotometry, that can measure the individual sizes of thousands of particles with nanometric accuracy in just a few minutes. This new method, based on the measurement of the scattering amplitude of each nanoparticle, overcomes some of the limitations observed in previous works and theoretically allows the characterization of nanoparticles of any size with a simple extra calibration step. As proof of concept, we characterized thousands of spherical nanoparticles of different sizes. This new method shows excellent accuracy, with less than a 3% discrepancy in direct comparison with transmission electron microscopy. Although the effectiveness of this method has been demonstrated with spherical nanoparticles, its real strength lies in its adaptability to more complex geometries by using an alternative analytical method to the one described here.

Electric, magnetic, and shear field-directed assembly of inorganic nanoparticles

Ordered assemblies of inorganic nanoparticles (NPs) have shown tremendous potential for wide applications due to their unique collective properties, which differ from those of individual NPs. Various assembly methods, such as external field-directed assembly, interfacial assembly, template assembly, biomolecular recognition-mediated assembly, confined assembly, and others, have been employed to generate ordered inorganic NP assemblies with hierarchical structures. Among them, the external field-directed assembly method is particularly fascinating, as it can remotely assemble NPs into well-ordered superstructures. Moreover, external fields (e.g., electric, magnetic, and shear fields) can introduce a local and/or global field intensity gradient, resulting in an additional force on NPs to drive their rotation and/or translation. Therefore, the external field-directed assembly of NPs becomes a robust method to fabricate well-defined functional materials with the desired optical, electronic, and magnetic properties, which have various applications in catalysis, sensing, disease diagnosis, energy conversion/storage, photonics, nano-floating-gate memory, and others. In this review, the effects of an electric field, magnetic field, and shear field on the organization of inorganic NPs are highlighted. The methods for controlling the well-ordered organization of inorganic NPs at different scales and their advantages are reviewed. Finally, future challenges and perspectives in this field are discussed.

A Metal Nanoparticle Thermistor with the Beta Value of 10 000 K

The thermistor, typically made from metallic oxides, is a type of resistor whose electrical resistance is dependent on its temperature. Despite the wide usage, the limitations of ceramic thermistors become increasingly apparent as devices with improved performances are sought and as new applications emerge. Herein, a thermistor that is showed with a beta (B) value of 10 000 K can be made exclusively from metal nanoparticles functionalized with charged organic ligands. This B value is hard to achieve for ceramic devices, which is due to the increase of effective counterion concentration and its mobility upon thermal activation. Importantly, the performance of the nanoparticle thermistor is maintained when it is fabricated on a flexible substrate and experiences reversible bending. Demos of thermistor arrays for heat transfer, distribution, and comparison of their performance with commercial products are also demonstrated. Owing to the low temperature and simple casting process, conformably flexible characteristics, stable solid states, and ultra-high sensitivities, this device is expected to be practically used soon.

Polygonal gold nanocrystal induced efficient phase transition in 2D-MoS2for enhancing photo-electrocatalytic hydrogen generation

Plasmonic nanocrystals (NCs) assisted phase transition of two-dimensional molybdenum disulfide (2D-MoS₂) unlashes numerous opportunities in the fields of energy harvesting via electrocatalysis and photoelectrocatalysis by enhancing electronic conductivity, increasing catalytic active sites, lowering Gibbs free energy for hydrogen adsorption and desorption, etc. Here, we report the synthesis of faceted gold pentagonal bi-pyramidal (Au-PBP) nanocrystals (NC) for efficient plasmon-induced phase transition (from 2 H to 1 T phase) in chemical vapor deposited 2D-MoS₂. The as-developed Au-PBP NC with the increased number of corners and edges showed an enhanced multi-modal plasmonic effect under light irradiations. The overpotential of hydrogen evolution reaction (HER) was reduced by 61 mV, whereas the Tafel slope decreased by 23.7 mV/dec on photoexcitation of the Au-PBP@MoS₂hybrid catalyst. The enhanced performance can be attributed to the light-induced 2H to 1 T phase transition of 2D-MoS₂, increased active sites, reduced Gibbs free energy, efficient charge separation, change in surface potential, and improved electrical conductivity of 2D-MoS₂film. From density functional theory (DFT) calculations, we obtain a significant change in the electronic properties of 2D-MoS₂(i.e. work function, surface chemical potential, and the density of states), which was primarily due to the plasmonic interactions and exchange-interactions between the Au-PBP nanocrystals and monolayer 2D-MoS₂, thereby enhancing the phase transition and improving the surface properties. This work would lay out finding assorted routes to explore more complex nanocrystals-based multipolar plasmonic NC to escalate the HER activity of 2D-MoS₂and other 2D transition metal dichalcogenides.

Recent advances in metal-based nanoporous materials for sensing environmentally-related biomolecules

Highly sensitive, stable, selective, efficient, and short reaction time sensors play a substantial role in daily life/industry and are the need of the day. Due to the rising environmental issues, nanoporous carbon and metal-based materials have attracted significant attention in environmental analysis owing to their intriguing and multifunctional properties and cost-effective and rapid detection of different analytes by sensing applications. Environmental-related issues such as pollution have been a significant threat to the world. Therefore, it is necessary to fabricate highly promising performance-based sensor materials with excellent reliability, selectivity and good sensitivity for monitoring various analytes. In this regard, different methods have been employed to fabricate these sensors comprising metal, metal oxides, metal oxide carbon composites and MOFs leading to the formation of nanoporous metal and carbon composites. These composites have exceptional properties such as large surface area, distinctive porosity, and high conductivity, making them promising candidates for several versatile sensing applications. This review covers recent advances and significant studies in the sensing field of various nanoporous metal and carbon composites. Key challenges and future opportunities in this exciting field are also part of this review.

In Situ TEM Observation of the Atomic Transport Process during the Coalescence of Au Nanoparticles

In practical applications, the coalescence of metal nanoparticles (NPs) is a major factor affecting their physical chemistry properties. Currently, due to a lack of understanding of the atomic-level mechanisms during the nucleation and growth stages of coalescence, the correlation between the different dynamic factors in the different stages of NP coalescence is unclear. In this study, we used advanced in situ characterization techniques to observe the formation of atomic material transport channels (Au chains) during the initiation of coalescence nucleation. We focused on the movement and migration states of Au atoms and discovered an atomic ordered arrangement growth mechanism that occurs after the completion of nucleation. Simultaneously, we used density functional theory to reveal the formation principle of Au chains. These findings improve our understanding of the atomic-scale coalescence process, which can provide a new perspective for further research on coalescence atomic dynamics.

Mixed-Ligand gold nanoparticles based optical sensor array for the recognition and quantification of seven toxic metals

Sensor arrays use pattern recognition for the identification and quantification of analytes. In the presented work, a gold nanoparticle (GNP) based optical sensor array was employed to classify and quantify seven toxic metals (arsenic, barium, cadmium, cerium, chromium, lead, and mercury). The sensor array receptors were GNPs functionalized by mercaptoundecanoic acid, 2-mercaptoethanesulfonate, and a 1:1 mixture of the two ligands. The mixed-ligand particle responds to the same analytes as the mono-ligand particles but in a distinctive way. This behavior demonstrates the high potential of mixed-ligand particles in the fabrication of sensor array receptors. The responses of the GNPs to different concentrations of the seven metal ions were analyzed, and a unique "classification trajectory" was produced for every metal. Samples of different metal concentrations were then measured and identified using the "classification trajectories". Once sample composition has been identified, a PLSR model, produced from the concatenated sensor array spectra of four calibration samples for each nanoparticle, was used to determine the metal concentration.

Site-selective modification of metallic nanoparticles

Surface patterning of inorganic nanoparticles through site-selective functionalization with mixed-ligand shells or additional inorganic material is an intriguing approach to developing tailored nanomaterials with potentially novel and/or multifunctional properties. The unique physicochemical properties of such nanoparticles are likely to impact their behavior and functionality in biological environments, catalytic systems, and electronics applications, making it vital to understand how we can achieve and characterize such regioselective surface functionalization. This Feature Article will review methods by which chemists have selectively modified the surface of colloidal nanoparticles to obtain both two-sided Janus particles and nanoparticles with patchy or stripey mixed-ligand shells, as well as to achieve directed growth of mesoporous oxide materials and metals onto existing nanoparticle templates in a spatially and compositionally controlled manner. The advantages and drawbacks of various techniques used to characterize the regiospecificity of anisotropic surface coatings are discussed, as well as areas for improvement, and future directions for this field.

Graphene Nanobeacons with High-Affinity Pockets for Combined, Selective, and Effective Decontamination and Reagentless Detection of Heavy Metals

Access to clean water for drinking, sanitation, and irrigation is a major sustainable development goal of the United Nations. Thus, technologies for cleaning water and quality-monitoring must become widely accessible and of low-cost, while being effective, selective, sustainable, and eco-friendly. To meet this challenge, hetero-bifunctional nanographene fluorescent beacons with high-affinity pockets for heavy metals are developed, offering top-rated and selective adsorption for cadmium and lead, reaching 870 and 450 mg g^-1 , respectively. The heterobifunctional and multidentate pockets also operate as selective gates for fluorescence signal regulation with sub-nanomolar sensitivity (0.1 and 0.2 nm for Pb²⁺ and Cd²⁺ , respectively), due to binding affinities as low as those of antigen-antibody interactions. Importantly, the acid-proof nanographenes can be fully regenerated and reused. Their broad visible-light absorption offers an additional mode for water-quality monitoring based on ultra-low cost and user-friendly reagentless paper detection with the naked-eye at a limit of detection of 1 and 10 ppb for Pb²⁺ and Cd²⁺ ions, respectively. This work shows that photoactive nanomaterials, densely-functionalized with strong, yet selective ligands for targeted contaminants, can successfully combine features such as excellent adsorption, reusability, and sensing capabilities, in a way to extend the material's applicability, its life-cycle, and value-for-money.

Urchin-like magnetic microspheres for cancer therapy through synergistic effect of mechanical force, photothermal and photodynamic effects

Background

Magnetic materials mediated by mechanical forces to combat cancer cells are currently attracting attention. Firstly, the magnetic force penetrates deeper into tissues than the NIR laser alone to destroy tumours. Secondly, the synergistic effect of nano-magnetic-material characteristics results in a viable option for the targeted killing of cancer cells. Therefore, mechanical force (MF) produced by magnetic nanomaterials under low frequency dynamic magnetic field combined with laser technology is the most effective, safe and efficient tool for killing cancer cells and tumour growth.

Results

In this study, we synthesized novel urchin-like hollow magnetic microspheres (UHMMs) composed of superparamagnetic Fe₃O₄. We demonstrated the excellent performance of UHMMs for killing laryngocarcinoma cancer cells through mechanical force and photothermal effects under a vibrating magnetic field and near-infrared laser, respectively. The killing efficiency was further improved after loading the synthesised UHMMs with Chlorin e6 relative to unloaded UHMMs. Additionally, in animal experiments, laryngocarcinoma solid tumour growth was effectively inhibited by UHMMs@Ce6 through magneto-mechanic force, photothermal and photodynamic therapy.

Conclusions

The biocompatibility and high efficiency of multimodal integrated therapy with the UHMMs prepared in this work provide new insights for developing novel nano therapy and drug loading platforms for tumour treatment. In vivo experiments further demonstrated that UHMMs/Ce6 are excellent tools for strongly inhibiting tumour growth through the above-mentioned characteristic effects.

Graphical Abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s12951-022-01411-y.

Quantitative Surface-Enhanced Spectroscopy

Surface-enhanced Raman scattering (SERS), a powerful technique for trace molecular detection, depends on chemical and electromagnetic enhancements. While recent advances in instrumentation and substrate design have expanded the utility, reproducibility, and quantitative capabilities of SERS, some challenges persist. In this review, advances in quantitative SERS detection are discussed as they relate to intermolecular interactions, surface selection rules, and target molecule solubility and accessibility. After a brief introduction to Raman scattering and SERS, impacts of surface selection rules and enhancement mechanisms are discussed as they relate to the observation of activation and deactivation of normal Raman modes in SERS. Next, experimental conditions that can be used to tune molecular affinity to and density near SERS substrates are summarized and considered while tuning these parameters are conveyed. Finally, successful examples of quantitative SERS detection are discussed, and future opportunities are outlined. Expected final online publication date for the Annual Review of Physical Chemistry, Volume 73 is April 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Field-effect-transistor-based ion sensors: ultrasensitive mercury(II) detection via healing MoS2 defects

The contamination of water with heavy metal ions represents a harsh environmental problem resulting from societal development. Among various hazardous compounds, mercury ions (Hg²⁺) surely belong to the most poisonous ones. Their accumulation in the human body results in health deterioration, affecting vital organs and eventually leading to chronic diseases, and, in the worst-case scenario, early death. High selectivity and sensitivity for the analyte of choice can be achieved in chemical sensing using suitable active materials capable of interacting at the supramolecular level with the chosen species. Among them, 2D transition metal dichalcogenides (TMDCs) have attracted great attention as sensory materials because of their unique physical and chemical properties, which are highly susceptible to environmental changes. In this work, we have fabricated MoS₂-based field-effect transistors (FETs) and exploited them as platforms for Hg²⁺ sensing, relying on the affinity of heavy metal ions for both point defects in TMDCs and sulphur atoms in the MoS₂ lattice. X-ray photoelectron spectroscopy characterization showed both a significant reduction of the defectiveness of MoS₂ when exposed to Hg²⁺ with increasing concentration and a shift in the binding energy of 0.2 eV suggesting p-type doping of the 2D semiconductor. The efficient defect healing has been confirmed also by low-temperature photoluminescence measurements by monitoring the attenuation of defect-related bands after Hg²⁺ exposure. Transfer characteristics in MoS₂ FETs provided further evidence that Hg²⁺ acts as a p-dopant of MoS₂. Interestingly, we observed a strict correlation of doping with the concentration of Hg²⁺, following a semi-log trend. Hg²⁺ concentrations as low as 1 pM can be detected, being way below the limits imposed by health regulations. Electrical characterization also revealed that our sensor can be efficiently washed and used multiple times. Moreover, the developed devices displayed a markedly high selectivity for Hg²⁺ against other metal ions as ruled by soft/soft interaction among chemical systems with appropriate redox potentials, being a generally applicable approach to develop chemical sensing devices combining high sensitivity, selectivity and reversibility, to meet technological needs.

Study the plasmonic property of gold nanorods highly above damage threshold via single-pulse spectral hole-burning experiments

Intense femtosecond laser irradiation reshapes gold nanorods, resulting in a persistent hole in the optical absorption spectrum of the nanorods at the wavelength of the laser. Single-pulse hole-burning experiments were performed in a mixture of nanorods with a broad absorption around 800 nm with a 35-fs laser with 800 nm wavelength and 6 mJ/pulse. A significant increase in hole burning width at an average fluence of 10⁶ J/m² has been found, suggesting a tripled damping coefficient of plasmon. This shows that the surface plasmonic effect still occurs at extremely high femtosecond laser fluences just before the nanorods are damaged and the remaining 10% plasmonic enhancement of light is at the fluence of 10⁶ J/m², which is several orders of magnitude higher than the damage threshold of the gold nanorods. Plasmon-photon interactions may also cause an increase in the damping coefficient.

Wearable chem-biosensing devices: from basic research to commercial market

Wearable chem-biosensors have been garnering tremendous interest due to the significant potential in tailored healthcare diagnostics and therapeutics. With the development of the medical diagnostics revolution, wearable chem-biosensors as a rapidly emerging wave allow individuals to perform on-demand detection and obtain the required in-depth information. In contrast to commercial wearables, which tend to be miniaturized for measuring physical activities, the recent progressive wearable chem-biosensing device have mainly focused on non-invasive or minimally invasive monitoring biomarkers at the molecular level. Wearables is a multidisciplinary subject, and chem-biosensing is one of the most significant technologies. In this review, the currently basic academic research of wearable chem-biosensing devices and its commercial transformation were summarized and highlighted. Moreover, some representative wearable products on the market for individual health managements are presented. Strategies for the identification and sensing of biomarkers are discussed to further promote the development of wearable chem-biosensing devices. We also shared the limitations and breakthroughs of the next generation of chemo-biosensor wearables, from home use to clinical diagnosis.

Polarization-dependent plasmonic nano-tweezer as a platform for on-chip trapping and manipulation of virus-like particles

Plasmonics advanced materials with the capability of environmental variation detection extend the application of visible light, satisfying the demands of less footprint, easy-to-use, and high tunability for visible-sensitive materials. In this area, localized surface plasmon can significantly enhance the electric field smaller than the radiation wavelength providing a strong gradient force required in optical tweezing systems. Owing to this beneficial advantage over the conventional optical tweezers, the plasmonic tweezers have matured to a stage where today become increasingly attractive for trapping and manipulation of nanoparticles. In this work, we are proposing a tunable plasmonic nano-tweezer that operates according to the polarization of excitation laser light. The proposed novel structure encompasses a gold nano-ring and a gold nano-disk in which by changing the beam polarization, the hot spots position also changes, and the nanoparticles can be relocated around the circumference of a circle trapped at a certain point. According to 3D finite-difference-time-domain simulations, the proposed nano-tweezing system can potentially exert a trapping force of 164 pNW-1 on a 20 nm HIV-1 virus-like particle.

Harnessing selectivity in chemical sensing via supramolecular interactions: from functionalization of nanomaterials to device applications

Chemical sensing is a strategic field of science and technology ultimately aiming at improving the quality of our lives and the sustainability of our Planet. Sensors bear a direct societal impact on well-being, which includes the quality and composition of the air we breathe, the water we drink, and the food we eat. Pristine low-dimensional materials are widely exploited as highly sensitive elements in chemical sensors, although they suffer from lack of intrinsic selectivity towards specific analytes. Here, we showcase the most recent strategies on the use of (supra)molecular interactions to harness the selectivity of suitably functionalized 0D, 1D, and 2D low-dimensional materials for chemical sensing. We discuss how the design and selection of receptors via machine learning and artificial intelligence hold a disruptive potential in chemical sensing, where selectivity is achieved by the design and high-throughput screening of large libraries of molecules exhibiting a set of affinity parameters that dictates the analyte specificity. We also discuss the importance of achieving selectivity along with other relevant characteristics in chemical sensing, such as high sensitivity, response speed, and reversibility, as milestones for true practical applications. Finally, for each distinct class of low-dimensional material, we present the most suitable functionalization strategies for their incorporation into efficient transducers for chemical sensing.

A New Spin-Correlated Plasmon in Novel Highly Oriented Single-Crystalline Gold Quantum Dots

A concept of spin plasmon, a collective mode of spin-density, in strongly correlated electron systems has been proposed since the 1930s. It is expected to bridge between spintronics and plasmonics by strongly confining the photon energy in the subwavelength scale within single magnetic-domain to enable further miniaturizing devices. However, spin plasmon in strongly correlated electron systems is yet to be realized. Herein, we present a new spin correlated-plasmon at room temperature in novel Mott-like insulating highly oriented single-crystalline gold quantum-dots (HOSG-QDs). Interestingly, the spin correlated-plasmon is tunable from the infrared to visible, accompanied by spectral weight transfer yielding a large quantum absorption midgap state, disappearance of low-energy Drude response, and transparency. Supported with theoretical calculations, it occurs due to an interplay of surprisingly strong electron-electron correlations, s-p hybridization and quantum confinement in the s band. The first demonstration of the high sensitivity of spin correlated-plasmon in surface-enhanced Raman spectroscopy is also presented.

Highly Sensitive Colorimetric Naked-Eye Detection of HgII Using a Sacrificial Metal-Organic Framework

This study has developed a specific, easy, and novel approach to designing a sacrificial metal-organic framework (MOF) that can detect and measure the amount of Hg²⁺ in aqueous and nonaqueous solutions using the naked eye. The functionalized [Zn(oba)(RL3)_0.5]_n·1.5DMF (TMU-59) provides the ability of simple visual assessment or colorimetric readout without sophisticated analytical equipment. Because of the special interaction with Hg²⁺, degradation of the structure of this unique MOF causes the solution to change color from colorless to a pink that is easily recognizable to the naked eye. The presence of a methyl group plays a major role in naked-eye detection by a qualitative sensor. Furthermore, this qualitative sensor data for the production of a simple, instant, and portable red, green, and blue (RGB)-based quantitative sensor were used to determine the concentration of Hg²⁺ in different specimens. As a turn-off fluorescence sensor, this unique structure is also capable of detecting Hg²⁺ at very low concentrations (the limit of detection is 0.16 ppb). To the best of our knowledge, TMU-59 is the first MOF-based naked-eye sensor that can successfully and specifically display the presence of Hg²⁺ through a major color change.

Stereoselective Growth of Small Molecule Patches on Nanoparticles

Patchy nanoparticles featuring tunable surface domains with spatial and chemical specificity are of fundamental interest, especially for creating three-dimensional (3D) colloidal structures. Guided assembly and regioselective conjugation of polymers have been widely used to manipulate such topography on nanoparticles; however, the processes require presynthesized specialized polymer chains and elaborate assembly conditions. Here, we show how small molecules can form 3D patches in aqueous environments in a single step. The patch features (e.g., size, number, conformation, and stereoselectivity) are modulated by a self-polymerizable aromatic dithiol and comixed ligands, which indicates an autonomous assembly mechanism involving covalent polymerization and supramolecular assembly. Moreover, this method is independent of the underlying nanoparticle material and dimension, offering a streamlined and powerful toolset to design heterogeneous patches on the nanoscale.

Counterion Gradients around Charged Metal Nanoparticles Enabling Basic Electronics without Semiconductors

In modern electronics, metals have not occupied the same role as semiconductors because their electrical properties are largely independent of the potential that is applied to them. However, this limitation of bulk metals can be overcome at the nanoscale, where metal nanoparticles functionalized with charged organic ligands can have highly tunable electrical characteristics enabling the fabrication of basic electronic components. Here, we show the recent progress on the design and construction of the basic electronic components (e.g., diodes and transistors) based on charged metal nanoparticles and the coupled transport of ionic and electronic charges within nanoparticle layers (Poisson and Nernst-Planck diffusion equations, PNP model) and how to assemble these electronic components and various metal nanoparticle sensors to achieve basic computations and "chemoelectronics". Meanwhile, we envision the future research directions and a possible breakthrough in metal nanoparticle electronics.

Chemical sensing with Au and Ag nanoparticles

Noble metal nanoparticles (NPs) are ideal scaffolds for the fabrication of sensing devices because of their high surface-to-volume ratio combined with their unique optical and electrical properties which are extremely sensitive to changes in the environment. Such characteristics guarantee high sensitivity in sensing processes. Metal NPs can be decorated with ad hoc molecular building blocks which can act as receptors of specific analytes. By pursuing this strategy, and by taking full advantage of the specificity of supramolecular recognition events, highly selective sensing devices can be fabricated. Besides, noble metal NPs can also be a pivotal element for the fabrication of chemical nose/tongue sensors to target complex mixtures of analytes. This review highlights the most enlightening strategies developed during the last decade, towards the fabrication of chemical sensors with either optical or electrical readout combining high sensitivity and selectivity, along with fast response and full reversibility, with special attention to approaches that enable efficient environmental and health monitoring.

Metallic-Nanoparticle-Based Sensing: Utilization of Mixed-Ligand Monolayers

The last two decades have seen great advancements in fundamental understanding and applications of metallic nanoparticles stabilized by mixed-ligand monolayers. Identifying and controlling the organization of multiple ligands in the nanoparticle monolayer has been studied, and its effect on particle properties has been examined. Mixed-ligand protected particles have shown advantages over monoligand protected particles in fields such as catalysis, self-assembly, imaging, and drug delivery. In this Review, the use of mixed-ligand monolayer protected nanoparticles for sensing applications will be examined. This is the first time this subject is examined as a whole. Mixed-ligand nanoparticle-based sensors are revealed to be divided into four groups, each of which will be discussed. The first group consists of ligands that work cooperatively to improve the sensors' properties. In the second group, multiple ligands are utilized for sensing multiple analytes. The third group combines ligands used for analyte recognition and signal production. In the final group, a sensitive, but unstable, functional ligand is combined with a stabilizing ligand. The Review will conclude by discussing future challenges and potential research directions for this promising subject.

Nanostructured surfaces from ligand-protected metal nanoparticles

Nanostructuring surfaces with metal atoms or clusters represents a promising approach to create materials with unique electronic/magnetic properties and improved chemical reactivity. By means of plasma sputtering and mass spectrometric techniques, the deposition of precisely size-selected clusters onto single-crystal surfaces has been applied to prepare surfaces with tailored properties. However, nanostructured surfaces can as well be prepared with metal nanoparticles via solution-phase methods, but the difference is that nanoparticles prepared by wet chemistry are usually coated with a layer of ligands, which are essential not only for maintaining the size and the atomic structure of metallic cores, but also for playing crucial roles in the synthesis, physicochemical properties and catalytic activities of the nanoparticles. This Frontier covers aspects of nanostructured surfaces from ligand-protected metal nanoparticles, starting with high-resolution imaging of the ligand-protected metal nanoparticles, followed by periodic patterning of metal nanoparticles on surfaces and the well-controlled atomic layer deposition with nanoclusters at the liquid/solid interface. We also highlight the potential of the surface-supported structures from ligand-protected metal nanoparticles, and the challenges remaining to be tackled.

Polymer-Ligated Nanocrystals Enabled by Nonlinear Block Copolymer Nanoreactors: Synthesis, Properties, and Applications

The past several decades have witnessed substantial advances in synthesis and self-assembly of inorganic nanocrystals (NCs) due largely to their size- and shape-dependent properties for use in optics, optoelectronics, catalysis, energy conversion and storage, nanotechnology, and biomedical applications. Among various routes to NCs, the nonlinear block copolymer (BCP) nanoreactor technique has recently emerged as a general yet robust strategy for crafting a rich diversity of NCs of interest with precisely controlled dimensions, compositions, architectures, and surface chemistry. It is notable that nonlinear BCPs are unimolecular micelles, where each block copolymer arm of nonlinear BCP is covalently connected to a central core or polymer backbone. As such, their structures are static and stable, representing a class of functional polymers with complex architecture for directing the synthesis of NCs. In this review, recent progress in synthesizing NCs by capitalizing on two sets of nonlinear BCPs as nanoreactors are discussed. They are star-shaped BCPs for producing 0D spherical nanoparticles, including plain, hollow, and core-shell nanoparticles, and bottlebrush-like BCPs for creating 1D plain and core/shell nanorods (and nanowires) as well as nanotubes. As the surface of these NCs is intimately tethered with the outer blocks of nonlinear BCPs used, they can thus be regarded as polymer-ligated NCs (i.e., hairy NCs). First, the rational design and synthesis of nonlinear BCPs via controlled/living radical polymerizations is introduced. Subsequently, their use as the NC-directing nanoreactors to yield monodisperse nanoparticles and nanorods with judiciously engineered dimensions, compositions, and surface chemistry is examined. Afterward, the intriguing properties of such polymer-ligated NCs, which are found to depend sensitively on their sizes, architectures, and functionalities of surface polymer hairs, are highlighted. Some practical applications of these polymer-ligated NCs for energy conversion and storage and drug delivery are then discussed. Finally, challenges and opportunities in this rapidly evolving field are presented.

A General Hypoxia-Responsive Molecular Container for Tumor-Targeted Therapy

Enhanced drug delivery can improve the therapeutic efficacy of drugs and help overcome side effects. However, many reported drug-delivery systems are too complex and irreproducible for practical use. In this work, the design of a hypoxia-responsive molecular container based on calixarene, called CAC4A, which presents a significant advance in practical, hypoxia-targeted drug-delivery, is reported. CAC4A enables a wide variety of clinical drugs to be quantitatively loaded to improve their solubility and stability, as well as enable the administration of reduced doses. Furthermore, as a result of its azo functional groups, which are sensitive to reduction within a hypoxic environment, it is possible to achieve tumor-targeted drug-release with reduced side effects. CAC4A fulfils all essential requirements for a drug-delivery system in addition to multiple advantages, including facile preparation, well-defined molecular weight, and structure, and universal applicability. Such features collectively enable supramolecular prodrugs to be formulated simply and reproducibly, with potential for bench-to-bedside translation. Moreover, CAC4A is amenable to other therapy modalities and can be facilely decorated with functional groups and hybridized with nanomaterials, providing ample possibilities for its role in future drug-delivery systems.

Programmable ROS-Mediated Cancer Therapy via Magneto-Inductions

Reactive oxygen species (ROS), a group of oxygen derived radicals and derivatives, can induce cancer cell death via elevated oxidative stress. A spatiotemporal approach with safe and deep-tissue penetration capabilities to elevate the intracellular ROS level is highly desirable for precise cancer treatment. Here, a mechanical-thermal induction therapy (MTIT) strategy is developed for a programmable increase of ROS levels in cancer cells via assembly of magnetic nanocubes integrated with alternating magnetic fields. The magneto-based mechanical and thermal stimuli can disrupt the lysosomes, which sequentially induce the dysfunction of mitochondria. Importantly, intracellular ROS concentrations are responsive to the magneto-triggers and play a key role for synergistic cancer treatment. In vivo experiments reveal the effectiveness of MTIT for efficient eradication of glioma and breast cancer. By remote control of the force and heat using magnetic nanocubes, MTIT is a promising physical approach to trigger the biochemical responses for precise cancer treatment.

Molecular Features for Probing Small Amphiphilic Molecules with Self-Assembled Monolayer-Protected Nanoparticles

The sensing of small molecules poses the challenge of developing devices able to discriminate between compounds that may be structurally very similar. Here, attention has been paid to the use of self-assembled monolayer (SAM)-protected gold nanoparticles since they enable a modular approach to tune single-molecule affinity and selectivity simply by changing functional moieties (i.e., covering ligands), along with multivalent molecular recognition. To date, the discovery of monolayers suitable for a specific molecular target has relied on trial-and-error approaches, with ligand chemistry being the main criterion used to modulate selectivity and sensitivity. By using molecular dynamics, we showcase that either individual molecular characteristics and/or collective features such as ligand flexibility, monolayer organization, ligand local ordering, and interfacial solvent properties can also be exploited conveniently. The knowledge of the molecular mechanisms that drive the recognition of small molecules on SAM-covered nanoparticles will critically expand our ability to manipulate and control such supramolecular systems.

Miscible-Solvent-Assisted Two-Phase Synthesis of Monolayer-Ligand-Protected Metal Nanoclusters with Various Sizes

Effective yet versatile synthetic strategies for size-tunable metal nanoclusters (NCs) are scarce. This has hampered the development of this unique class of nanomaterials. Here, a general protocol is reported for the synthesis of high-quality metal NCs protected by a variety of organic ligands (e.g., selenolate, thiolate, and phosphine) based on a miscible-solvent-assisted phase transfer between water and organic solution. This method is demonstrated to be facile, rapid (≤3 h), scalable (gram-scale), and versatile. The size of the selenolated and thiolated Au NCs can be tuned from Au₁₀ to Au₆₁ by simply varying the miscible solvent in proportions and types. The advantages of this method, such as quick phase separation and no need for purification treatment, enable real-time monitoring of metal NC growth within the NaBH₄ reduction system. The results show that the size of Au NCs gradually increases with increasing valence electron count by a stepwise 2x e^- hopping mechanism (x = 0-5), i.e., 0 e^- → 2 e^- → 4 e^- → 8 e^- → 18 e^- → 22 e^- → 32 e^- .

Remote Control of Mechanical Forces via Mitochondrial-Targeted Magnetic Nanospinners for Efficient Cancer Treatment

In cells, mechanical forces play a key role in impacting cell behaviors, including adhesion, differentiation, migration, and death. Herein, a 20 nm mitochondria-targeted zinc-doped iron oxide nanocube is designed as a nanospinner to exert mechanical forces under a rotating magnetic field (RMF) at 15 Hz and 40 mT to fight against cancer. The nanospinners can efficiently target the mitochondria of cancer cells. By means of the RMF, the nanocubes assemble in alignment with the external field and produce a localized mechanical force to impair the cancer cells. Both in vitro and in vivo studies show that the nanospinners can damage the cancer cells and reduce the brain tumor growth rate after the application of the RMF. This nanoplatform provides an effective magnetomechanical approach to treat deep-seated tumors in a spatiotemporal fashion.

Polymer-guided assembly of inorganic nanoparticles

The self-assembly of inorganic nanoparticles is of great importance in realizing their enormous potentials for broad applications due to the advanced collective properties of nanoparticle ensembles. Various molecular ligands (e.g., small molecules, DNAs, proteins, and polymers) have been used to assist the organization of inorganic nanoparticles into functional structures at different hierarchical levels. Among others, polymers are particularly attractive for use in nanoparticle assembly, because of the complex architectures and rich functionalities of assembled structures enabled by polymers. Polymer-guided assembly of nanoparticles has emerged as a powerful route to fabricate functional materials with desired mechanical, optical, electronic or magnetic properties for a broad range of applications such as sensing, nanomedicine, catalysis, energy storage/conversion, data storage, electronics and photonics. In this review article, we summarize recent advances in the polymer-guided self-assembly of inorganic nanoparticles in both bulk thin films and solution, with an emphasis on the role of polymers in the assembly process and functions of resulting nanostructures. Precise control over the location/arrangement, interparticle interaction, and packing of inorganic nanoparticles at various scales are highlighted.

3D hybrid networks of gold nanoparticles: mechanoresponsive electrical humidity sensors with on-demand performances

We have engineered macroscopic 3D porous networks of gold nanoparticles (AuNPs) chemically interconnected by di-thiolated ethylene glycol oligomers. The formation of such superstructures has been followed by means of UV-Vis spectroscopy by monitoring the aggregation-dependent plasmonic band of such nanomaterials. The controlled chemical tethering of the AuNPs with di-thiolated linkers possessing a well-defined contour length rules the interparticle distance. The use of ad-hoc linkers ensures charge transport via direct tunneling and the hygroscopic nature of the ethylene glycol backbone allows interaction with moisture. Upon interaction with water molecules from the atmosphere, our 3D networks undergo swelling reducing the tunnelling current passing through the system. By exploiting such a behavior, we have devised a new approach for the fabrication of electrical resistive humidity sensors. For the first time we have also introduced a new strategy to fabricate stable and robust devices by covalently attaching our 3D networks to gold electrodes. Devices comprising both 4 (TEG) or 6 (HEG) ethylene glycol repetitive units combined with AuNPs exhibited (i) unprecedentedly high response speed (∼26 ms), (ii) short recovery time (∼250 ms) in the absence of any hysteresis effect, and (iii) a linear response to humidity changes characterized by a highest sensitivity of 51 kΩ per RH(%) for HEG- and 500 Ω per RH(%) for TEG-based devices. The employed green solution processing in water and the extreme robustness of our 3D networks make them interesting candidates for the fabrication of sensors which can operate under extreme conditions and for countless cycles.

Charged Metal Nanoparticles for Chemoelectronic Circuits

Although metal nanoparticles (NPs) stabilized with self-assembled monolayers (SAMs) of various organic ligands have proven useful in applications ranging from chemical sensing, to bionanotechnology, to plasmonics and energy conversion, they have not been widely considered as suitable building blocks of electronic circuitry, largely because metals screen electric fields and prevent electrically tunable conductivity. However, when metal nanoparticles a few nanometers in size are stabilized by charged ligands and placed under bias, the counterions surrounding the NPs can redistribute and establish local electric fields that feed back into the electronic currents passing through the nanoparticles' metallic cores. Herein, the manner in which the interplay between counterion gradients and electron flows can be controlled by using different types of SAMs is discussed. This can give rise to a new class of nanoparticle-based "chemoelectronic" logic circuits capable of sensing, processing, and ultimately reporting various chemical signals.

Logic-Gate-Actuated DNA-Controlled Receptor Assembly for the Programmable Modulation of Cellular Signal Transduction

Programming cells to sense multiple inputs and activate cellular signal transduction cascades is of great interest. Although this goal has been achieved through the engineering of genetic circuits using synthetic biology tools, a nongenetic and generic approach remains highly demanded. Herein, we present an aptamer-controlled logic receptor assembly for modulating cellular signal transduction. Aptamers were engineered as "robotic arms" to capture target receptors (c-Met and CD71) and a DNA logic assembly functioned as a computer processor to handle multiple inputs. As a result, the DNA assembly brings c-Met and CD71 into close proximity, thus interfering with the ligand-receptor interactions of c-Met and inhibiting its functions. Using this principle, a set of logic gates was created that respond to DNA strands or light irradiation, modulating the c-Met/HGF signal pathways. This simple modular design provides a robust chemical tool for modulating cellular signal transduction.

Multicolor Aptasensor Based on DNA-Induced Au-Ag Nanorods for Simultaneous and Visual Detection of Inorganic and Organic Mercury

Compared to inorganic mercury (Hg²⁺), methyl-mercury (CH₃Hg⁺) and ethyl-mercury (C₂H₅Hg⁺) (organic mercury) not only have a much stronger toxicity but also are more easily accumulated by marine organisms to produce bioamplification. Therefore, the simultaneously onsite detection of Hg²⁺ and organic mercury is of great significance to ensure the safety of seafood, and it is also a hard challenge. We designed a T-rich aptamer, H_T7, for specifically recognizing Hg²⁺ and organic mercury and developed a multicolor aptasensor for simultaneous discrimination and detection of Hg²⁺ and organic mercury with only bare-eye observation using H_T7 as a recognition probe and gold nanorods (AuNRs) as a signal. In the presence of Hg²⁺ and Ag⁺, Hg²⁺ preferentially and specifically bind with H_T7 immobilized on AuNRs surface and induce the formation of a monolayer Ag/Hg amalgam on the AuNRs surface after reduction, resulting in a change in color from orange to faint purple and a corresponding shift in the absorption peak from 820 to 730 nm in the solution. However, in the presence of CH₃Hg⁺ or C₂H₅Hg⁺ and Ag⁺, CH₃Hg⁺ or C₂H₅Hg⁺ preferentially bind with H_T7 immobilized on the AuNRs surface and induce the formation of a monolayer Ag⁰ on the AuNRs surface after reduction, which results in the change in color from orange to atrovirens and the corresponding shift in the absorption peak shift from 820 to 670 nm in the solution. Thus, the inorganic and organic mercury (total of CH₃Hg⁺ and C₂H₅Hg⁺) can be specifically discriminated and detected by only bare-eye observation. The method can be used to simultaneously detect inorganic and organic mercury in seawater by the bare-eye observation with a visual detection limit of 2.0 ppm for Hg²⁺ and 10.0 ppm for organic mercury. The success of this study is a useful enlightenment to develop an instrument-free method for an onsite detection of trace inorganic and organic mercury in environment by a bare-eye observation, although the sensitivity of the method is relatively low.

A Unique Interactive Nanostructure Knitting based Passive Sampler Adsorbent for Monitoring of Hg2+ in Water

This work reports the development of ultralight interwoven ultrathin graphitic carbon nitride (g-CN) nanosheets for use as a potential adsorbent in a passive sampler (PAS) designed to bind Hg²⁺ ions. The g-CN nanosheets were prepared from bulk g-CN synthesised via a modified high-temperature short-time (HTST) polycondensation process. The crystal structure, surface functional groups, and morphology of the g-CN nanosheets were characterised using a battery of instruments. The results confirmed that the as-synthesized product is composed of few-layered nanosheets. The adsorption efficiency of g-CN for binding Hg²⁺ (100 ng mL⁻¹) in sea, river, rain, and Milli-Q quality water was 89%, 93%, 97%, and 100%, respectively, at natural pH. Interference studies found that the cations tested (Co²⁺, Ca²⁺, Zn²⁺, Fe²⁺, Mn²⁺, Ni²⁺, Bi³⁺, Na⁺, and K⁺) had no significant effect on the adsorption efficiency of Hg²⁺. Different parameters were optimised to improve the performance of g-CN such as pH, contact time, and amount of adsorbent. Optimum conditions were pH 7, 120 min incubation time and 10 mg of nanosheets. The yield of nanosheets was 72.5%, which is higher compared to other polycondensation processes using different monomers. The g-CN sheets could also be regenerated up to eight times with only a 20% loss in binding efficiency. Overall, nano-knitted g-CN is a promising low-cost green adsorbent for use in passive samplers or as a transducing material in sensor applications.

Nanotransducers for Near-Infrared Photoregulation in Biomedicine

Photoregulation, which utilizes light to remotely control biological events, provides a precise way to decipher biology and innovate in medicine; however, its potential is limited by the shallow tissue penetration and/or phototoxicity of ultraviolet (UV)/visible light that are required to match the optical responses of endogenous photosensitive substances. Thereby, biologically friendly near-infrared (NIR) light with improved tissue penetration is desired for photoregulation. Since there are a few endogenous biomolecules absorbing or emitting light in the NIR region, the development of molecular transducers is essential to convert NIR light into the cues for regulation of biological events. In this regard, optical nanomaterials able to convert NIR light into UV/visible light, heat, or free radicals are suitable for this task. Here, the recent developments of optical nanotransducers for NIR-light-mediated photoregulation in medicine are summarized. The emerging applications, including photoregulation of neural activity, gene expression, and visual systems, as well as photochemical tissue bonding, are highlighted, along with the design principles of nanotransducers. Moreover, the current challenges and perspectives in this field are discussed.

Poly(adenine)-mediated DNA-functionalized gold nanoparticles for sensitive detection of mercury ions in aqueous media

In this work, a facile and sensitive colorimetric sensor for Hg2+ ions based on poly (adenine)-mediated DNA-functionalized gold nanoparticles (Au NPs) is reported. One DNA sequence consisting of poly-A and T-rich DNA was designed rationally. Poly-A was used as an anchoring block to bind tightly to Au NPs, and T-rich DNA was utilized for specific recognition of Hg2+ ions. With the assistance of poly-A, T-rich DNA was easily introduced onto the surface of Au NPs and kept an upright orientation. In the presence of Hg2+ ions, T base binding with Hg2+ ions results in the formation of "T-Hg2+-T" among the Au NPs, which caused aggregation of the Au NPs and a subsequent change in the color of the solution, from wine red to grayish blue. On this occasion, the limit of detection (LOD) was 3.75 nM Hg2+ ions with a linear range from 5 nM to 200 nM, as measured by UV-Vis spectroscopy. Moreover, successful application of this method for the detection of Hg2+ ions in real samples was demonstrated.

Predicting the effect of chain-length mismatch on phase separation in noble metal nanoparticle monolayers with chemically mismatched ligands

Nanoparticles (NPs) protected with a ligand monolayer hold promise for a wide variety of applications, from photonics and catalysis to drug delivery and biosensing. Monolayers that include a mixture of ligand types can have multiple chemical functionalities and may also self-assemble into advantageous patterns. Previous work has shown that both chemical and length mismatches among these surface ligands influence phase separation. In this work, we examine the interplay between these driving forces, first by using our previously-developed configurationally-biased Monte Carlo (CBMC) algorithm to predict, then by using our matrix assisted laser desorption/ionization mass spectrometry (MALDI-MS) technique to experimentally probe, the surface morphologies of a series of two-ligand mixtures on the surfaces of ultrasmall silver NPs. Specifically, we examine three such mixtures, each of which has the same chemical mismatch (consisting of a hydrophobic alkanethiol and a hydrophilic mercapto-alcohol), but varying degrees of chain-length mismatch. This delicate balance between chemical and length mismatches provides a challenging test for our CBMC prediction algorithm. Even so, the simulations are able to quantitatively predict the MALDI-MS results for all three ligand mixtures, while also providing atomic-scale details from the equilibrated ligand structures, such as patch sizes and co-crystallization patterns. The resulting monolayer morphologies range from randomly-mixed to Janus-like, demonstrating that chain-length modifications are an effective way to tune monolayer morphology without needing to alter chemical functionalities.

Characterization of mixed-ligand shells on gold nanoparticles by transition metal and supramolecular surface probes

Two simple colorimetric and fluorimetric methods to probe the surface of gold nanoparticles.

Bimodal atomic force microscopy for the characterization of thiolated self-assembled monolayers

Surface coatings are becoming an integral part of materials. In recent years, molecular coatings have found larger acceptance and uses. Among them, self-assembled monolayers (SAMs) are attractive due to their inherent versatility, manufacturability, and scale up ease. Understanding their structure-properties relationships in realistic conditions remains a major challenge. Here we present a methodology based on simultaneous topographical and nanomechanical characterization of SAMs using a commercially available setup for bimodal atomic force microscopy (AFM). It allows for accurate and quantitative measurement of surface elasticity, which is correlated to molecular ordering through topographical imaging. Our results indicate that effective surface elasticity (E*) scales with monolayer formation-time and ligand-length, parameters known to affect ligand ordering. The method developed, is extended to provide localization of the chemical species present in thiolated binary SAMs. Within the systems tested phase separation down to ∼10 nm domains could be observed both in the topography and in the elasticity channel.

Computational and Experimental Investigation of Janus-like Monolayers on Ultrasmall Noble Metal Nanoparticles

Detection of monolayer morphology on nanoparticles smaller than 10 nm has proven difficult with traditional visualization techniques. Here matrix-assisted laser desorption ionization mass spectrometry (MALDI-MS) is used in conjunction with atomistic simulations to detect the formation of Janus-like monolayers on noble metal nanoparticles. Silver metal nanoparticles were synthesized with a monolayer consisting of dodecanethiol (DDT) and mercaptoethanol (ME) at varying ratios. The nanoparticles were then analyzed using MALDI-MS, which gives information on the local ordering of ligands on the surface. The MALDI-MS analysis showed large deviations from random ordering, suggesting phase separation of the DDT/ME monolayers. Atomistic Monte Carlo (MC) calculations were then used to simulate the nanoscale morphology of the DDT/ME monolayers. In order to quantitatively compare the computational and experimental results, we developed a method for determining an expected MALDI-MS spectrum from the atomistic simulation. Experiments and simulations show quantitative agreement, and both indicate that the DDT/ME ligands undergo phase separation, resulting in Janus-like nanoparticle monolayers with large, patchy domains.