Nanopores provide a sensing volume comparable to the size of single molecules, which is conducive to realize single-molecule detection. Early nanopore-based sensors mainly depend on electrical measurements, single-molecule detection in nanopores using optical methods has recently emerged as an alternative approach. In this context, plasmonic nanopores have been proposed and extensively investigated. Plasmonic nanopores combine plasmonic nanoantennas with nanopores, which confine and enhance optical excitation near the nanopores, forming hotspots that have significant advantages for single-molecule sensing. First, the strong electromagnetic field enhances the interaction between the field and the single molecule in the hotspot, while reduces the noise generated by molecules at other locations. Second, the gradient electric field in the plasmonic nanopore attracts nearby molecules, increasing the molecular translocation time. Third, the refractive index distribution of the plasmonic nanopore directly affects the resonance conditions, which constitutes the molecular information in the hotspot. Therefore, plasmonic nanopores have been increasingly explored for single-molecule detection in recent years.

With the development of single-molecule optical detection based on plasmonic nanopores, diverse plasmonic nanopore structures and measurement schemes have been proposed to detect a variety of single molecules, which renders a review of the existing studies necessary.

Progress In this review, the typical structures of plasmonic nanopores and their mechanisms of field enhancement, the mostly used detection strategies, the application progress, and typical achievements in single-molecule detection are described.

Section 2 summarizes the classification and typical structures of plasmonic nanopores. Plasmonic nanopore structures can be divided into three types: plasmonic nanopores based on metal micro/nanostructures (including plasmonic nanowell-nanopore, plasmonic nanoslit cavity, and plasmonic bullseye structures), plasmonic nanopores based on nanogaps (including bowtie nanoantennas, double nanoholes, and nanoparticle dimers), and plasmonic nanopores based on glass nanopipettes (including gold nanoporous spheres and thin-film-modified single nanopores). The structure characteristics and field enhancement mechanisms of each plasmonic nanopore structures are discussed.

Section 3 describes the four mostly used detection strategies for plasmonic nanopores and their characteristics, including fluorescence detection, surface-enhanced Raman spectroscopy (SERS), surface plasmon resonance displacement sensing, and combined optical and electrical sensing. Fluorescence detection is one of the most widely used methods for the detection of single molecules in solution. Plasmonic nanopores significantly amplify the fluorescence signal and decrease background noise. However, the introduction of fluorescent dyes may affect the molecular state of single molecules, increasing the experimental complexity. The development of label-free single-molecule detection strategies such as SERS and plasmonic resonance displacement sensing is highly desired. Meanwhile, combined optical and electrical sensing is expected to break the current bottleneck of electrical signal detection in solid nanopores, enabling single-molecule sensing with high signal-to-noise ratio and high resolution.

Section 4 provides a brief review of the application progress and typical achievements of plasmonic nanopores in the detection of single molecules including DNA, proteins, peptides, and other bioparticles. The detection of DNA molecules mainly includes DNA molecular translocation, DNA identification, and gene sequencing. Plasmonic structures such as bowtie nanoantennas, bullseyes, and plasmonic nanowells-nanoholes have been used for the translocation detection of single DNA molecules, and the optical label-free detection of single DNA molecules has been achieved using plasmon resonance displacement sensing. DNA identification and sequencing processes are mainly based on SERS signals. Due to the weak spectral signals of single DNA molecules, plasmonic nanopores providing larger field enhancement are preferred for SERS detection. The detection of proteins and peptides involves capture, identification, and structural characterization. In addition to the detection of biological single molecules, plasmonic nanopores are used for the detection of bioparticles such as bacteria and viruses.

Section 5 summarizes the opportunities and challenges for further research and application of plasmonic nanopores.

Conclusions and Prospects Plasmonic nanopores realize nanometer-sized hotspot fields near the nanopores, enhance the interaction between the field and matter, and achieve highly sensitive detection with high spatial resolution. In recent years, plasmonic nanopores have been increasingly explored for the optical detection of single molecules. Optical detection offers many advantages over electrical sensing strategies, such as decoupling of the driving voltage and buffer conditions from the signal strength and big-bandwidth data acquisition. As one of the most promising optical strategies, SERS based on plasmonic nanopores provides compositional information on the detected substance. Extending the residence time of the detected substance near the nanopores is the main challenge to realize plasmonic-nanopore-based SERS. This is currently achieved through the optical force of plasmonic nanopores. Although some progress has been reported, it is still necessary to develop multiphysics coupling models to achieve a more controllable and stable three-dimensional manipulation of molecules. Combining optical and electrical sensing strategies in plasmonic nanopores has also great potential for superior single-molecule detection. Plasmonic nanoantennas optimize the performance of electrical measurements by increasing the molecular translocation time and enhancing electrical signals. Further development of the above technologies will promote the plasmonic nanopore technology for single-molecule detection and identification and realize the single-molecule sequencing technology.

Single-molecule detection promotes the development of biological research because it reveals details that remain concealed to ensemble measurements. However, the detection and analysis of single molecules confront great challenges. On the one hand, single molecules produce very weak signals, requiring extremely sensitive detection methods. On the other hand, the sizes of single molecules are mainly confined to the nanoscale and they move with the naturally random state. It is necessary to isolate the detected signal of the target molecule and exclude interferences from other molecules. Therefore, the development of single-molecule detection methods attracts considerable researchers attention.