Nanoscale Horizons blog

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.

Electrifying the synthesis of commodity chemicals can play a critical role in achieving carbon neutrality, as well as addressing global energy and environmental problems. Among the diverse range of chemicals, hydrogen peroxide (H₂O₂) has emerged as a promising green oxidant and liquid hydrogen carrier. However, H₂O₂ is currently produced by the anthraquinone autooxidation process under harsh conditions with huge energy consumption. Consequently, researchers have been actively exploring alternative approaches for the synthesis of H₂O₂ using renewable resources under milder conditions.

In this regard, extensive studies have focused on the green synthesis of H₂O₂ using renewable electricity and employing either electrocatalysts or photocatalysts directly powered by sunlight. Most conventional studies on electrochemical H₂O₂ production have been conducted under alkaline conditions, which are known to facilitate efficient H₂O₂ production. However, it is important to note that H₂O₂ becomes unstable at high pHs. Moreover, from an environmental standpoint, there is a strong desire to develop electrocatalysts that can operate at neutral pH.

In this context, a recent paper by Yang et al. reports very interesting results. The researchers prepared graphitic carbon nitride (g-C₃N₄) nanosheets (CNNS) embedded with various transition metal single atoms (TM SAs) and discovered that TM SA-embedded CNNS show high electrocatalytic activity for H₂O₂ production at neutral pHs. Among the various TM SAs tested, Ni SAs on CNNS were particularly effective and showed the highest mass-specific activity of ∼503 mmol g_cat⁻¹ h⁻¹ and H₂O₂ selectivity of ~98%. According to their mechanistic analysis, the introduction of TM SA promotes the formation of N-C=N sites, which are beneficial for H₂O₂ production via a two-electron oxygen reduction reaction (2e⁻ ORR), while suppressing the formation of C-C/C=C sites, which are beneficial for H₂O production via a 4e⁻ ORR. This suggests the excellent function of g-C₃N₄ as a support for TM SAs in selectively producing H₂O₂.

Fig. 1 Schematic of H2O2 production from H2O and O2 on a transition metal embedded graphitic carbon nitride sheet. Reproduced from DOI: 10.1039/D2NH00564F with permission from the Royal Society of Chemistry.

Notably, this paper is also intriguing from an academic perspective, as it demonstrates the efficient use of g-C₃N₄ as a support material for electrocatalysts, deviating from its traditional application as a photocatalyst in conventional studies. The findings offer new insights into the potential of g-C₃N₄ in catalytic systems and open avenues for further research in the field of sustainable chemical synthesis.

To find out more, please read:

Transition metal single atom-optimized g-C₃N₄ for the highly selective electrosynthesis of H₂O₂ under neutral electrolytes
Hongcen Yang, Fei Ma, Niandi Lu, Shuhao Tian, Guo Liu, Ying Wang, Zhixia Wang, Di Wang, Kun Tao, Hong Zhang and Shanglong Peng
Nanoscale Horiz., 2023, 8, 695–704

About the blogger

Jungki Ryu is a ​P​rofessor at ​Ulsan National Institute of Science and Technology (UNIST) and member of the Nanoscale Horizons Advisory Board. Prof. Ryu’s research focuses on developing innovative electrochemical and photoelectrochemical systems using nanomaterials​ for hydrogen production, CO2 conversion and biomass/waste utilization. You can follow Jungki on Twitter @jungki1981

Drug delivery and targeted treatment of diseases is one of the prominent focus areas of recent research. Development of new therapeutic approaches involving novel drug delivery materials (e.g., nanomaterials) requires validation that these materials do not affect the existing properties of the cellular environment. Now, researchers from the Third Military Medical University (China) and National University of Singapore (Singapore) have found that DNA-nanostructure-based drug delivery vehicles do not affect the cellular environment as previously thought, but in fact aid in restoring endothelial leakiness in vascular diseases.

For proper cellular function, endothelial barriers maintain vascular permeability by which essential nutrients and oxygen reaches the target tissues. Several diseases and inflammations cause endothelial leakiness, which in turn leads to disease progress and ineffective treatments. Now, researchers use cell and mouse models of pulmonary arterial hypertension (PAH), a lung disease, to demonstrate that DNA-based therapeutic carriers can effectively restore the endothelial barrier. They developed a triangular DNA structure and loaded small-interfering RNA (siRNA) molecules that target specific disease-associated genes. In this case, the researchers targeted the Atg101 gene that causes autophagy and in turn affects endothelial leakiness. They found that the siRNA-loaded DNA carriers were taken up by cells and reduced endothelial gaps to 0.3% compared to untreated cells that showed 10% endothelial gaps, thus providing a 30-fold improvement. This treatment was specific to the siRNA cargo loaded in the structure. When they used a random siRNA sequence loaded on to the DNA structures, there was no improvement in endothelial gaps. The group then tested the siRNA-loaded DNA structures in mice and found that the drug-loaded DNA structures provided protection against right ventricular and pulmonary artery dysfunction, a promising step forward to creating a treatment strategy for such diseases.

Fig. 1 (A) Design of DNA aptamer and Atg101 siRNA (siAtg101) conjugated DNA nanostructures. DNA aptamers are positioned either at the protruding points (DTA-V1) or the corners of the structure core (DTA-V2). (B) Aptamer-decorated DNA nanostructures bind to HPAECs and are subsequently internalized (C and D). The DNA nanostructures might be internalized through aptamer-mediated endocytosis. The embedded siRNA takes effect and restores endothelial integrity similar to the reversal of “NanoEL”. Reproduced from DOI: 10.1039/D2NH00348A with permission from the Royal Society of Chemistry.

This study provides new information on how nanomaterials interact with biological systems and affect cellular environment such as endothelial leakiness which is typically associated with tumor regions. As DNA structures could successfully delivery siRNA molecules to suppress endothelial leakiness related to a vascular disease, this study opens up the possibility of using DNA-based drug delivery carriers in therapeutics approaches beyond just cancer.

To find out more, please read:

Attenuating endothelial leakiness with self-assembled DNA nanostructures for pulmonary arterial hypertension
Qian Liu, Di Wu, Binfeng He, Xiaotong Ding, Yu Xu, Ying Wang, Mingzhou Zhang, Hang Qian, David Tai Leong and Guansong Wang
Nanoscale Horiz., 2023, 8, 270–278

About the blogger

Arun Richard Chandrasekaran is a Senior Research Scientist at The RNA Institute at the University at Albany, State University of New York, and member of the Nanoscale Horizons Community Board. Dr Chandrasekaran’s research lab focusses on using DNA as a material to build nanoscale structures, with applications in drug delivery, data storage and crystallography. You can follow Arun on Twitter @arunrichardc