Liquid-phase exfoliation (LPE) is a cheap, scalable and facile way to produce graphene nanosheets. However, what is gained in processability, is then lost in homogeneity of the resulting nanomaterial. This issue is particularly relevant in devices for which inter-nanosheet resistance plays a significant role in their intrinsic performance. For instance, piezoresistive nanosheet-based strain sensors have already been shown to be deeply influenced by network composition and morphology.

In this recent work by Caffrey et al., the influence of nanosheet thickness was investigated and correlated to the piezoresistance of printed graphene sensors. Firstly, the nanosheet suspension was prepared via LPE, different flake sizes were selected through liquid cascade centrifugation (LCC) and their thickness was then estimated via atomic force microscopy (AFM). The team produced different sensors via spray coating a network of graphene nanosheets with thicknesses between 3 and 20 nm (Fig. 1).

Fig 1. (A) Photograph of a printed sensor. (B) Raman spectra of sprayed graphene films inks of different size-selected nanosheets. (C)–(E) SEM images of networks composed of nanosheets with different size ranges. Reproduced from DOI: 10.1039/D4NH00224E with permission from the Royal Society of Chemistry.

The devices show a clear trend of increasing resistivity and gauge factor with increasing thickness. By using a simple model that correlates the network resistivity with nanosheet thickness, a new model that successfully correlates the gauge factor with thickness was obtained (Fig. 2).

Fig 2. Plot of gauge factor as a function of nanosheet thickness showing both experimental data and the model fitting. Reproduced from DOI: 10.1039/D4NH00224E with permission from the Royal Society of Chemistry.

The authors carefully analyzed the different contributions to the gauge factor and were able to differentiate between the effect of straining of the nanosheets themselves and of the inter-nanosheet junctions. From fitting this model, they observed that, interestingly, strain has a significant influence on nanosheet resistivity, which means that applied strain not only makes the flakes in the network slide on each other, but also induces a detectable deformation on the flakes themselves. Unexpectedly, the calculated nanosheets’ gauge factor was negative. This was attributed to a decrease in nanosheet strain with applied strain, possibly due to the relaxation of built-in strain during network formation or the release of point-to-basal plane contacts.

By applying a theoretical approach to experimental data, the authors were able to interpret and quantify the piezoresistive response in disordered graphene networks produced by LPE. Overall, this study presents a step forward in actively understanding the mechanisms behind the piezoresistive behavior of printed graphene sensors.

To find out more, please read:

Quantifying the effect of nanosheet dimensions on the piezoresistive response of printed graphene nanosheet networks
Eoin Caffrey, Jose M. Munuera, Tian Carey and Jonathan N. Coleman
Nanoscale Horiz., 2024, Advance Article

About the blogger

Sara Domenici is a PhD student at Politecnico di Torino (Turin, Italy) under the supervision of Prof. Teresa Gatti. She was born in Verona (Italy) in 1998. In 2020, she obtained her Bachelor’s degree in Chemistry at the University of Padova (Italy). In 2022, she completed the Double Degree Programme between the University of Padova and the Justus-Liebig University in Giessen (Germany), where she spent 12 months, and obtained a Master’s Degree in Chemistry. Her PhD project is focused on Janus two-dimensional materials for energy conversion, but she also works on hydrogel sensors and dye-sensitized solar cells (DSSCs).

The performance of nanomaterials is intricately linked to their size and combination form. Quantum dots (QDs), for instance, exhibit unique electronic and quantum properties due to their size effect. Moreover, the optical and electronic attributes of QD assemblies can be tailored by adjusting the size of individual QDs and their spatial arrangement within the assembly. Thus, it is imperative to explore suitable and universally applicable methods to prepare adjustable QDs and their assemblies.

In a recent study, Akter et al. devised mesoscopic QD assemblies using a novel bio-catalytic nanoparticle shaping (BNS) approach. Specifically, the authors employed L-lysine as a linker to assemble CdSe/CdS QDs, initially yielding ultra-large QD assemblies. Subsequently, these assemblies were catalytically cleaved by trypsin, resulting in mesoscopic QD assemblies (ms-QD) with a size of 84 nm (Fig. 1A). Relative to single QDs, the redshift and weakened emission observed in the photoluminescence spectrum of ms-QD suggest the presence of internal emission reabsorption processes within the assembly, which holds promise for leveraging energy or electron/hole transfer processes in the production of new optical materials.

Fig 1. (A) Synthetic pathway of ms-QD by the BNS method and corresponding TEM image, and (B) the synthetic scheme of organic molecule assemblies. Reproduced from DOI: 10.1039/D4NH00134F with permission from the Royal Society of Chemistry.

Additionally, the authors synthesized and investigated nanoassemblies of the organic molecule tetrakis(4-carboxyphenyl)porphyrin (Fig. 1B) using a similar method. They observed significant differences in reactive oxygen generation under light among the various assemblies, indicating the potential to modulate the molecular function of the particle’s core unit by altering the composition of connecting molecules.

In summary, this method demonstrates high versatility and can be employed for preparing assemblies of both QDs and organic molecules. Furthermore, it allows for the replacement of biological enzymes/substrates as needed to generate various nanoassemblies with unique physicochemical properties for further applications.

To find out more, please read:

Bio-catalytic nanoparticle shaping for preparing mesoscopic assemblies of semiconductor quantum dots and organic molecules
Rumana Akter, Nicholas Kirkwood, Samantha Zaman, Bang Lu, Tinci Wang, Satoru Takakusagi, Paul Mulvaney, Vasudevanpillai Biju and Yuta Takano
Nanoscale Horiz., 2024, Advance Article

About the blogger

Chao Wang is a postdoctoral fellow at Oregon State University and a member of the Nanoscale Horizons Community Board. His research focuses on multiple nanomedicines (especially metal-organic frameworks) and their theranostics for cancer, endometriosis and stem cell tracking.

Currently, nanomaterials (NM) are attracting significant attention in the field of biomedicine. However, once these nanomaterials are utilized for in vivo treatments they interact with the surrounding physiological environment, leading to the adsorption of various biomolecules onto their surfaces, forming a biomolecular corona (BMC) and thereby influencing the performance and behavior of the nanomaterials. Presently, the in vitro studies of NM primarily involve dispersing the nanoparticles in 10% fetal bovine serum (FBS) and then evaluating their toxicity and therapeutic effects. However, this evaluation method is insufficient as it cannot accurately simulate the conditions of human blood. Moreover, this practical issue remains unresolved to date.

Fig 1. Schematic illustrating the molecular and biological biases arising from the well-known in vitro/in vivo mismatch in nanomedicine due to the biomolecular corona. Reproduced from DOI: 10.1039/D3NH00510K with permission from the Royal Society of Chemistry.

To validate the series of biases existing in established experimental practices and to advance the fields of nanomedicine and nanotoxicology, this study investigated two NM types with vastly different physicochemical properties commonly used in biomedicine. The research compared the molecular and biological biases resulting from the mismatch between NM dispersed in 10% FBS (utilized for in vitro biological assays) and whole human plasma (HP, closer to in vivo administration schemes). Through comparative analysis using proteomics, lipidomics, high-throughput multi-parameter in vitro screening, and single-molecule feature analysis, it was demonstrated that the dynamic changes in BMC composition are material dependent and that cell viability, transport pathways, and autophagic cascades are influenced by the presence or absence of pre-formed BMC corona. These findings underscore the potential limitations of NM in vitro testing in accurately representing real in vivo conditions. Therefore, it is necessary to establish new shared protocols to enhance the accuracy and predictive capability of NM testing.

In summary, this study confirms the biases that may exist when using standard in vitro conditions for NM toxicology assessments, reminding us of the need to establish a comprehensive experimental framework to generate and support new knowledge in the field of biologically relevant nanomaterial interactions. For instance, integrating advanced predictive tools such as artificial intelligence and machine learning will enable nanotoxicology and nanomedicine to progress towards personalized solutions for precision healthcare.

To find out more, please read:

Sources of biases in the in vitro testing of nanomaterials: the role of the biomolecular corona
Valentina Castagnola, Valeria Tomati, Luca Boselli, Clarissa Braccia, Sergio Decherchi, Pier Paolo Pompa, Nicoletta Pedemonte, Fabio Benfenati and Andrea Armirotti
Nanoscale Horiz., 2024, Advance Article

About the blogger

Fangfang Cao is a Research Fellow at National University of Singapore and a member of the Nanoscale Horizons Community Board. Dr Cao’s research focuses on nanocatalytic medicine and microbial therapy.

Face-centered cubic (fcc) metals, such as Au and Ag, usually adopt a packed crystal structure in bulk. However, the equilibrium structure could differ when only a handful of atoms compose a nanocluster. Theories have predicted that particles less than a few nanometers would favor a decahedral packing with a five-fold symmetry; when even fewer atoms are present, say less than two hundred, a 20-fold icosahedral packing would become the lowest-energy configuration. Such fluctuations of the nuclei/seeds may have played a critical role in defining the shape of colloidal nanoparticles in many wet chemical syntheses.

In recent work, a cross-institutional team led by Richard E. Palmer and Thomas J. A. Slater reported the direct observation of such fluctuations on a nearly second-by-second basis. The team synthesized Au nanoclusters containing 309±15 atoms on an amorphous carbon film through mass-selected magnetron sputtering. Subsequently, aberration-corrected scanning transmission electron microscopy (STEM) was employed to track the atomic structures of Au nanoclusters with a frame rate of 0.4–0.7 per second (Fig. 1). To identify the cluster type in each frame, the team compared them to a collection of simulated images with different cluster structures and tilt angles. The clusters exhibited highly dynamic switching between decahedral, icosahedral, and single-crystalline structures under the electron beam, which is sufficiently strong to overcome the energy barriers between such transitions.

Fig. 1 Au309±15 clusters fluctuating under the electron beam. High-angle annular dark field (HAADF) imaging on an aberration-corrected scanning transmission electron microscope (STEM) resolved the atomic structure of these Au nanoclusters frame by frame. Adapted from the supporting data DOI: 10.5281/zenodo.10522408, CC-BY 4.0.

Notably, the authors showed that the Au_309±15 clusters favor the decahedral structure the most, followed by icosahedral and then single-crystalline structures (Fig. 2a). This result is consistent with the probabilities obtained from a snapshot of an ensemble. In theory, the lower-energy structures would have a higher probability of appearance. The ranking of isomeric preferences observed in this study indicates that the cluster size is within a range where the energy ranks in fcc > icosahedral > decahedral (Fig. 2b). Taken together, this work illustrates the possibility of atomic-resolution electron microscopy, when combined with image simulations, to track the isomeric evolution of metal nanoclusters and may shed light on how we understand and regulate nanostructures with atomic precision.

Fig. 1 (a) Histogram of isomer abundances from dynamic movies compared with a static image of a cluster ensemble. Reproduced from DOI: 10.1039/D3NH00291H with permission from the Royal Society of Chemistry. (b) Schematic energy landscape of cluster structures for fcc metals. A red shade indicates the cluster size range in the current study. Ih: icosahedral. Dh: decahedral. Adapted from DOI: 10.1002/anie.202015166 with permission from Wiley-VCH.

To find out more, please read:

Frame-by-frame observations of structure fluctuations in single mass-selected Au clusters using aberration-corrected electron microscopy
Malcolm Dearg, Cesare Roncaglia, Diana Nelli, El Yakout El Koraychy, Riccardo Ferrando, Thomas J. A. Slater, and Richard E. Palmer
Nanoscale Horiz., 2024, 9, 143-147

About the blogger

Jingshan S. Du is a Washington Research Foundation Postdoctoral Fellow at Pacific Northwest National Laboratory and a member of the Nanoscale Horizons Community Board. His research spans crystal formation and transformation pathways, in situ electron microscopy, and hybrid organic/inorganic nanostructures. Du received a Ph.D. in Materials Science and Engineering from Northwestern University in 2021. At Northwestern, he worked on complex nanoparticle systems, correlative electron microscopy of hybrid nanostructures, and nanoscale thermodynamics. Du received a Certificate for Management for Scientists and Engineers from Northwestern’s Kellogg School of Management in 2021 and a B.Sc. in Engineering from Zhejiang University Chu Kochen Honors College in 2015. You can follow him on Twitter @JingshanDu. The views expressed in this article do not necessarily reflect those of the author’s employer or the US government.

In the last few years, immunotherapy has paved new paths for effective treatment of different cancers. Specifically, immunotherapy stimulates T cells, a type of white blood cell called lymphocytes that help to fight germs and destroy tumours. Immunotherapy can be used as a monotherapy or combined with chemotherapy and surgery. Unfortunately, cancer cells and their microenvironment have many sophisticated defence mechanisms that pose considerable challenges to immunotherapy effectiveness and progress. Current strategies to boost cancer immunotherapy include increasing the infiltration of T cells at the tumour site or blocking immune checkpoint-producing immune evasion.

In this regard, an exciting immunotherapy combination approach has been developed by Guixiang Xu and team based on an injectable hydrogel as a carrier to deliver a drug called linagliptin which is capable of inhibiting dipeptidyl peptidases 4 (DPP4) degradation. This leads to prolonged half-life of CXCL10 chemokines and thus, increases recruitment of T cells in the tumour site. Small molecule immune checkpoint blocker (BMS-202) particles were also loaded onto the developed drug carrier to block the programmed cell death-ligand (PD-L1), avoiding immune evasion. The team demonstrated that the application of hydrogel construct (S@LB) suppresses chemokine CXCL10 degradation, increasing T-cell infiltration, while BMS-202 particles inactivate PD-L1 checkpoint in vivo.

Fig. 1 Preparation and mechanism scheme of S@LB. (A) The preparation process of the S@LB solution. (B) Schematic illustration of an injectable hydrogel to reinforce cancer immunotherapy by promoting infiltration of T cells and regulating immune evasion. Reproduced from DOI: 10.1039/D3NH00401E with permission from the Royal Society of Chemistry.

The team tested the in vivo anti-tumour ability, immune response, and lung anti-metastatic effect of the S@LB in combination with chemotactic CXCL10 (S@LB + CXCL10). Their recent report shows that after 18 days of tumour removal, an immune memory effect was detected for the group treated with S@LB + CXCL10.

Overall, this study shows how nano-based hydrogel immunotherapy can be used as an innovative “weapon” against primary and distant tumours, along with efficient inhibition of lung metastasis, indicating tremendous potential for developing transformative clinical applications.

To find out more, please read:

Hydrogel-mediated tumor T cell infiltration and immune evasion to reinforce cancer immunotherapy
Guixiang Xu, Kai Liu, Xiangwu Chen, Yang Lin, Cancan Yu, Xinxin Nie, Wenxiu He, Nathan Karinc and  Yuxia Luan
Nanoscale Horiz., 2024, Advance Article

About the blogger

Susel Del Sol Fernández is a Marie Skłodowska-Curie Postdoctoral fellow at Aragon Nanoscience and Materials Institute (INMA-CSIC), Spain and a member of the Nanoscale Horizons Community Board. Dr Del Sol’s research focuses on designing smart functionalized magnetic nanoparticles for biomedical applications, including magnetic-optical hyperthermia treatment and magnetogenetics. You can follow her on X @SuselDelSol

The rapidly expanding energy and computing sectors are driving demand for high-performance semiconductor materials. Organic Semiconductors (OSCs) have emerged as attractive candidates for opto-electronic devices, thanks to their high carrier mobility, stability and tunability. However, accurate and independent quantification of charge carrier density and mobility has been an ongoing challenge in the OSC community. Working to overcome this challenge, recent research by Hermosilla-Palacios et al. presents a novel method for determining charge carrier characteristics in semiconducting single-walled carbon nanotubes (s-SWCNTs), a subtype of OSCs.

In this study, a nuclear magnetic resonance (NMR)-based approach is proposed to directly quantify charge carrier density and indirectly quantify carrier mobility (Fig. 1). The study puts forward a combined method utilizing ¹⁹F NMR and optical absorption measurements on s-SWCNTs in the presence of F-containing molecular dopants. The researchers demonstrated that changes in carrier density affect charge delocalization, resulting in a carrier density-dependent mobility, in contrary to that expected for mobility limited by ionized impurity scattering. This combined approach simplifies the measurement of carrier density in doped s-SWCNTs, constituting a valuable tool to the OSC community.

Fig. 1 (a) Cartoon showing the NMR tube sample composition: polymer dispersed s-SWCNT, excess polymer (in blue) and DDB-F72 molecules associated with a hole on the doped s-SWCNTs. Repeating unit of the polymer PF-PD is also presented for clarity. (b) Spectra corresponding to ¹⁹F NMR for neutral DDB-F72 dopant in d₈-toluene (6 mM, bottom), PF-PD polymer used to disperse s-SWCNTs with added DDB-F72 (6 mM, middle), and dispersed s-SWCNTs with added DDB-F72 (6 mM, top). Numbers show specific chemical shift. (c) Spectra corresponding to ¹⁹F NMR for doping series of s-SWCNT. Spectra are arbitrarily displaced along the y axis to show the different doping steps clearly. Lower dopant concentration (red) to higher dopant concentrations (blue). Reproduced from DOI: 10.1039/D3NH00480E with permission from the Royal Society of Chemistry.

While this study presents significant strides in measuring carrier density in s-SWCNTs, whether it can be effectively applied to a wide range of OSCs beyond s-SWCNTs remains to be seen. It should also be noted that the downfield shift observed with increasing dopant concentration may be complicated by factors other than charge delocalization of the hole distribution, such as dopant binding dynamics. The mechanistic origin of the chemical shift changes in the presence of dopants with NMR -active nuclei may refine our understanding of the local micro-environment around the redox-doped s-SWCNTs, prompting further investigations in this area.

In summary, this study develops an NMR-based method to quantify charge carrier density in s-SWCNTs and illustrates that the hole mobility in doped s-SWCNT networks increases with growing carrier density. The ability to tune, quantify, and optimize carrier density opens new avenues for applications such as photovoltaics, sensors, light-emitting diodes, field-effect transistors, and thermoelectric devices. The method’s potential applicability to various p-conjugated semiconductors using suitable NMR-active dopants makes it a versatile tool for the field. As the scientific community embraces this innovative approach, it heralds a new chapter in the design and development of high-performance semiconductor materials.

To find out more, please read:

Carrier density and delocalization signatures in doped carbon nanotubes from quantitative magnetic resonance
M. Alejandra Hermosilla-Palacios, Marissa Martinez, Evan A. Doud, Tobias Hertel, Alexander M. Spokoyny, Sofie Cambré, Wim Wenseleers, Yong-Hyun Kim, Andrew J. Ferguson and Jeffrey L. Blackburn
Nanoscale Horiz., 2024, Advance Article

About the blogger

Albert Liu is an Assistant Professor at the University of Michigan, and a member of the Nanoscale Horizons Community Board. Prof. Liu’s research group studies the effects of micro-confinement in nano-structured low dimensional materials, to address challenges in sustainability, robotics, and healthcare. You can follow Albert on Twitter @Albert_T_Liu

Chronic wounds are considered a major healthcare problem all over the world. Long-term infections and/or suppressed immune responses may cause chronic wounds with slower healing, resulting in increased mortality. Although antibiotics, skin disinfectants, and hydrogels are currently being used to combat microbial pathogenesis, they still have some significant limitations when used in clinical wound healing. Therefore, researchers have been exploring alternative approaches, such as combining antimicrobial, antioxidant, and anti-inflammatory agents for advanced chronic wound therapy.

Recently, nanomaterial-based antimicrobials have gained popularity thanks to catalytic and near-infrared (NIR) irradiation treatments, which induce controlled oxidative stress (photodynamic and catalytic therapies) and hyperthermia (photothermal therapies) to eradicate bacteria. However, little research into nanomaterial-based antimicrobial activity against biofilms and chronic wound healing in vivo has previously been reported.

Fig. 1 An overview of the properties of CuS/Qu–CNGs and their role in wound healing. Reproduced from DOI: 10.1039/D3NH00275F with permission from the Royal Society of Chemistry.

In this regard, an NIR-triggered multifunctional quercetin carbonized nanogel embedded with copper sulfide nanoclusters (CuS/Qu-CNG) was reported by Nain et al. for advanced therapy of chronic wounds. Polymerization and mild carbonation procedures were used to prepare quercetin carbonized nanogels (Qu-CNGs), which were subsequently used as templates to grow CuS in situ forming CuS/Qu-CNGs. The resulting CuS/Qu–CNGs are photoreactive and contain antioxidant and catalytic properties (oxidase- and peroxidase-like activities). As a result of their photo-responsive properties, CuS/Qu-CNGs significantly amplified their antimicrobial activity when exposed to NIR-II light. A CuS/Qu–CNGs MIC₉₀ value of 6–9 mg mL^-1 is ~125-fold lower than Qu or Qu–CNGs under NIR-II irradiation and was further improved by ~30-fold (ca. 0.2 mg mL^-1) in the presence of H₂O₂. Besides, CuS/Qu-CNGs demonstrated exceptional penetration ability, eliminating MRSA biofilms caused by diabetic wounds in diabetic mice. By suppressing pro-inflammatory cytokines (IL-1β) and boosting anti-inflammatory cytokines (IL-10 and TGF-β1), CuS/Qu-CNGs significantly accelerated wound healing by promoting angiogenesis, epithelialization and collagen synthesis. Finally, CuS/Qu–CNGs showed superior in vivo efficacy in treating bacterial infections and enhancing wound healing in diabetic mice.

In summary, a “Three in One” multifunctional CuS/Qu-CNGs with excellent antimicrobial/antioxidative/anti-inflammatory properties demonstrate great potential in treating bacterial infections and promoting chronic wound healing. This work is expected to provide new solutions for wound treatment complicated by microbial pathogenesis.

To find out more, please read:

NIR-activated quercetin-based nanogels embedded with CuS nanoclusters for the treatment of drug-resistant biofilms and accelerated chronic wound healing
Amit Nain, Yu-Ting Tseng, Akash Gupta, Yu-Feng Lin, Sangili Arumugam, Yu-Fen Huang, Chih-Ching Huang and Huan-Tsung Chang
Nanoscale Horiz., 2023, 8, 1652-1664

About the blogger

Jiangjiexing Wu is an Associate Professor at Tianjin University and a member of the Nanoscale Horizons Community Board. Dr Wu’s research focuses on the rational design and synthesis of functional nanomaterials (such as nanozymes) for analytical, biomedical, and environmental applications.

Recycling CO₂ molecules through photocatalysis represents an innovative technology to mitigate the emission of CO₂ gas. The deliberate introduction of defects into a photocatalyst plays a crucial role in optimizing photocatalytic performance, since the defects regulate the electronic structure, fine-tune selectivity by enhancing catalytic activity, and reduce the activation barrier of the catalyst.

A team of researchers based in Australia have recently developed a photocatalytic system for CO production by CO₂ reduction. In this study, defect-rich g-C₃N₄ serves as a semiconducting substrate, and it was further loaded with Ag nanoparticles (NPs) to act as a plasmonic source, resulting in the creation of the g-C₃N₄-Ag photocatalyst. The defects within the g-C₃N₄ were known as the active sites that enhance the efficiency of photocatalytic CO₂ conversion. In addition, these defects are strategically positioned alongside the loaded Ag NPs, which improves the effectiveness of injected hot electrons from the Ag NPs, thereby synergistically enhancing the activity of photocatalytic CO₂ reduction.

In the experiments involving defect-rich g-C₃N₄, different photodeposition times, including 10 minutes, 1 hour, 3 hours, and 5 hours, were performed to load Ag particles, denoted the as-preared photocatalyst as g-C₃N₄-Ag 10m, 1h, 3h, and 5h, respectively. These variants were evaluated for their photocatalytic performance in CO production via CO₂ reduction (Fig. 1a). The optimal performance was achieved with g-C₃N₄-Ag 1h, and the production of CO was confirmed through isotopic experiments (Fig. 1b). The g-C₃N₄-Ag photocatalysts were characterized using scanning transmission electron microscopy, as typical image is presented in Fig. 1c and the corresponding elemental mapping were presented in Fig. 1d to 1g.

Fig 1. (a) CO production rate based on various g-C3N4 based photocatalysts. (b) Isotope labelling experiments tested under 13CO2 and 12CO2, and the mass spectrometry signals at m/z = 28 and m/z = 29 are 13CO and 12CO, respectively. (c) STEM dark field image and (d)–(g) elemental mapping of g-C3N4-Ag 1h catalyst. Scale bar: 300 nm. Reproduced from DOI: 10.1039/D3NH00348E with permission from the Royal Society of Chemistry.

To elucidate the mechanism behind the defect engineering scenario, Ag NPs were initially loaded onto g-C₃N₄ via photodeposition. Due to the electron-rich environment of the point defects on g-C₃N₄, Ag⁺ ions selectively grow on these defect sites. The resulting g-C₃N₄-Ag composite was subsequently annealed. During this process, the new defects formed on the g-C₃N₄ substrate owing to the strain induced by the differing thermal expansion rates between the Ag and g-C₃N₄. These new defects were found to be located around the Ag NPs, representing a significant change in the pristine g-C₃N₄ following the introduction of Ag.

Furthermore, density functional theory (DFT) calculations were conducted to gain a deeper understanding of how the defects in g-C₃N₄ improve performance. Three models of photocatalysts were considered, including pristine g-C₃N₄, g-C₃N₄ with N vacancies, and N vacancies in g-C₃N₄ with O sites on the surface. In the models of pristine g-C₃N₄ and g-C₃N₄ with N vacancies, the formation of *COOH intermediates was identified as the rate-limiting step (RDS), and moreover, N vacancies in g-C₃N₄ were found to enhance the activity in this conversion (Fig 2a). For N vacancies in g-C3N4 with additional surface O sites (Fig. 2b), the initial reaction step favored the formation of *COOH intermediates from a thermodynamic perspective. Subsequently, the reduction of *COOH intermediates to *CO species occurred by reacting with protons, releasing H₂O molecules. In the case of O-enriched g-C₃N₄, this conversion became the RDS. DFT calculations indicated that the ΔG values for *COOH and *H to form *CO and H₂O at the C defect active sites were 0.96 eV, which determined the reaction rate (Fig. 2c). These results provide insight into the reasons behind the improved performance in CO production through CO₂ reduction.

Fig 2. Optimized configurations of reaction intermediates *COOH and *CO on the C atom and N vacancy active sites of (a) g-C3N4 with N vacancy and (b) N vacancies in g-C3N4 with O sites on the surface. (Red ball is oxygen atom, white ball is hydrogen atom, gray is carbon atom, and blue is nitrogen atom) (c) Gibbs free energy diagrams for photocatalytic CO2 reduction to CO on g-C3N4, g-C3N4 with N vacancy and O-occupied g-C3N4. Reproduced from DOI: 10.1039/D3NH00348E with permission from the Royal Society of Chemistry.

In summary, the deliberate introduction of active defects into g-C₃N₄ photocatalysts, strategically positioned near the plasmon centers of Ag NPs, optimizes the utilization efficiency of plasmonic hot electrons, resulting in an enhanced efficiency for CO₂ photoreduction. Importantly, this strategy has the potential for extension to various systems based on polymers, hard materials, and hybrid materials, offering promising applications that harness the functionalities of defects in a wide range of fields.

To find out more, please read the full article:

Defect engineering enhances plasmonic-hot electrons exploitation for CO₂ reduction over polymeric catalysts
Hang Yin, Zhehao Sun, Kaili Liu, Ary Anggara Wibowo, Julien Langley, Chao Zhang, Sandra E. Saji, Felipe Kremer, Dmitri Golberg, Hieu T. Nguyen, Nicholas Cox and Zongyou Yin
Nanoscale Horiz., 2023, Advance Article

About the blogger

Yuanxing Fang is a Professor at Fuzhou University, and a member of the Nanoscale Horizons Community Board. Prof. Fang’s research lab focuses on the synthesis of metal-free semiconductors for photoelectrochemical systems for energy and environmental applications, including water splitting, hydrogen peroxide synthesis, organic transformations and others.

Surface-enhanced Raman scattering (SERS) is known to be driven by two mechanisms: electromagnetic enhancement (e.g., plasmon excitation) and chemical enhancement (e.g., charge transfer). Although numerous SERS substrates have been reported and commercialized, distinguishing between these two mechanisms and controlling their contributions in real time remains a significant challenge. The challenge arises from the difficulty of accurately estimating the contribution of charge transfer and the limited ability to adjust SERS enhancement once the substrate has been prepared.

Now, a team of Chinese researchers have developed pyroelectric-responsive SERS substrates by combining a pyroelectric material, Pb(Mg,Nb)O₃-PbTiO₃ (PMN-PT), with plasmonic silver nanoparticles (Ag NPs). Their strategy takes advantage of the pyroelectric effect, which converts temperature fluctuations into electricity, thus modifying the charge on the surface of the SERS substrates (Fig. 1). Heating the substrates (dT/dt > 0) generates a downward electric field on the substrate surface, whereas cooling them (dT/dt < 0) generates an upward electric field. In both cases, the SERS signals can be significantly amplified due to the piezoelectric-induced charge transfer between the LUMO level of the analyte molecule and the Fermi level of Ag. During the heating and cooling processes, the intensity of SERS signals undergoes temporal changes, which can be modulated by adjusting the heating and cooling rate. Such chemical enhancement can further amplify SERS signals by over 100 times, compared to recordings obtained under steady temperature conditions based solely on plasmon excitation.

Fig. 1 Schematic depiction of the SERS substrate based on PMN-PT and Ag NPs, illustrating the signal enhancement during heating (dT/dt > 0), steady temperature (dT/dt = 0), and cooling (dT/dt < 0). Reproduced from DOI: 10.1039/D3NH00053B with permission from the Royal Society of Chemistry.

The researchers conducted systematic experimental characterizations and theoretical calculations to understand the SERS performance of these substrates in a variable temperature environment. Different analytes were used to demonstrate the universal applicability of this method. Density functional theory calculations were performed for the Ag NP-molecular system to reveal the redistribution of charge density in response to an upward or downward electric field. In order to verify the role of chemical enhancement, the researchers used a thin layer of aluminium oxide (Al₂O₃) as a barrier layer to prevent charge transfer between the Ag NPs and the analytes (Fig. 2). Overall, although the electromagnetic enhancement was not optimized in this strategy, the researchers provided an in-depth understanding of the SERS mechanism and the role of charge transfer in chemical enhancement.

Fig. 2 Schematic illustration of the SERS experiment setup for understanding the SERS enhancement mechanism of PMN-PT/Ag NPs by depositing a 5-nm Al₂O₃ layer to block the charge transfer between the Ag NPs and the analyte molecules. Reproduced from DOI: 10.1039/D3NH00053B with permission from the Royal Society of Chemistry.

Furthermore, the researchers successfully demonstrated a nanocavity structure with PMN-PT/Ag/Al₂O₃/Ag nanocubes (Ag NCs) (Fig. 3), which can be heated by simulated sunlight irradiation and achieve SERS enhancement, obviating the need for a temperature control platform. This development could have practical benefits for real-world applications.

Fig. 3 (a) The SERS measurement schematic diagram of PMN-PT/Ag/Al2O3/Ag NC substrate under simulated sunlight irradiation. (b) The temperature distribution images of PMN-PT/Ag/Al2O3/Ag NC after the simulated sunlight turned on and off. The SERS spectra of R6G (10^–7 M) (c) and CV (10^–7 M) (d) before and after simulated sunlight irradiation. Reproduced from DOI: 10.1039/D3NH00053B with permission from the Royal Society of Chemistry.

In summary, the novel combination of PMN-PT and Ag NPs allows for a straightforward observation of chemical SERS enhancement and its active tuning, both of which are traditionally challenging in this field. These findings will facilitate a deeper understanding of the SERS mechanism and the development of other SERS substrates to improve the detection sensitivity.

To find out more, please read:

Giant enhancement of the initial SERS activity for plasmonic nanostructures via pyroelectric PMN-PT
Mingrui Shao, Di Liu, Jinxuan Lu, Xiaofei Zhao, Jing Yu, Chao Zhang, Baoyuan Man, Hui Pan and Zhen Li
Nanoscale Horiz., 2023, 8, 948–957

About the blogger

Xiaolu Zhuo is an Assistant Professor at The Chinese University of Hong Kong, Shenzhen, and a member of the Nanoscale Horizons Community Board. Dr Zhuo’s research lab focuses on the synthesis of plasmonic and dielectric nanoparticles, their optical behaviors, and their applications.