Plasmonic paper for trace detection of analytes
The detection of trace levels of chemical and biological analytes has many important applications, including forensics and environmental monitoring. A highly promising approach to trace detection is based on the large enhancement of Raman scattering from molecules when they are adsorbed on or in close proximity to a nanostructured metal surface, known as surface-enhanced Raman scattering (SERS).1 Numerous SERS substrates—ranging from roughened metal films to highly ordered plasmonic nanostructure assemblies with nanometer-scale control over size, shape, and distance—have been fabricated using various lithographic and nonlithographic methods and tested for the trace-level detection of various analytes.
So far, most of these SERS substrates are based on rigid and brittle surfaces such as glass, silicon, and alumina. These substrates have been favored because of their compatibility with common micro- and nanofabrication technologies (e.g., photolithography and electron-beam lithography) and with well-established surface modification procedures (e.g., self-assembled monolayers) for anchoring the chemically synthesized plasmonic nanostructures on surfaces. However, fabrication and processing of these substrates is costly, and they preclude applications that require flexibility, such as swabbing the substrate on a surface of interest to make conformal contact for efficient sample collection.
Recently, we and others have introduced a novel SERS substrate based on conventional filter paper decorated with plasmonic nanostructures.2,3 Our work used pre-synthesized shape-controlled nanostructures, namely gold nanorods (AuNRs), which offer facile tunability of the localized surface plasmon resonance (LSPR) wavelength—that is, the wavelength of the collective electron oscillations (‘plasmons’) that occur in these structures on a material surface. We adsorbed the AuNRs on paper substrates by immersing the paper in an AuNR solution, followed by extensive rinsing to remove weakly adsorbed nanostructures. These plasmonic paper substrates exhibited excellent uniformity, as evidenced by scanning electron microscopy, LSPR spectra, and SERS spectra obtained across the substrate (see Figure 1).4 The relative standard deviation in SERS intensity following the exposure of plasmonic paper to model analytes such as 1,4-benzenedithiol (1,4-BDT) was ∼15%, which is remarkable considering the heterogeneous nature and inherent roughness of the paper substrates. As well as exhibiting a sub-nanomolar limit of detection, the plasmonic paper substrates could be employed as swabs to collect and detect trace amounts of analyte from real-world surfaces. Conformal contact and high SERS efficiency of the plasmonic paper enabled the detection of ∼100pg of 1,4-BDT spread over 2×2cm2: see Figure 1(D).
Some additional key functionalities are required, however, for these paper-based SERS substrates to analyze complex samples. In particular, separation, chemical selectivity, and pre-concentration of analytes are critical for realizing a versatile lab-on-chip platform. We must integrate these separation abilities into the paper. Although researchers have focused increasing efforts on extending the functionality of paper substrates, this work has usually involved lithographic processes or multilayer fabrication, which is achieved at the expense of the simplicity of the paper device. We therefore sought to design and develop a highly sensitive, label-free analytical platform that does not require any lithographic or microfabrication steps, while providing a multifunctional platform.
We have demonstrated a versatile approach that allows separation and pre-concentration of the different components of a complex sample in a small surface area by taking advantage of the properties of cellulose paper and the capillary effect, enhanced by the paper's shape.5 As a proof-of-principle, we cut paper substrates into star shapes and differentially functionalized the fingers with polyelectrolytes, which led to the separation of charged analytes on the paper's surface (see Figure 2). Furthermore, the enhanced capillary effect caused the vertices of the star-shaped paper to serve as pre-concentration sites for analytes deposited at the center of the star. A remarkable sub-attomolar detection of a model analyte (2-napathalenethiol) was achieved at the micrometer-scale detection spot, that is, at the fine tips of the paper.
The translation of SERS to real-world settings faces another often-overlooked challenge, namely the complexity of the chemical environment from which a target analyte has to be detected. To transform SERS into a viable detection platform in real-world settings, overcoming the inherently poor chemical selectivity of plasmonic nanostructures is critical. We have introduced a generic biomimetic approach, in which heterofunctional biological recognition elements (BREs) are integrated with plasmonic nanostructures to realize a chemically selective SERS substrate.6 We demonstrated this approach by employing a BRE peptide to detect trinitrotoluene (TNT): see Figure 3. The BRE comprises a TNT-binding moiety, a cysteine-terminal (which enables strong and stable binding of the peptide to AuNRs through the gold-sulfur bond), and a glycine spacer, which provides conformational flexibility and extends the TNT-binding moiety away from the AuNR surface to facilitate efficient binding of TNT by avoiding steric hindrance.6 The molecular specificity of this BRE enabled the selective detection of trace quantities of TNT in a chemically complex medium (shampoo solution), with a limit of detection of ~100 pM. Remarkably, no signs of chemical contamination were observed.
Plasmonic paper is thus rapidly emerging as a highly promising analytical platform for trace detection of chemical and biological analytes.7 The material's simplicity in fabrication and usage combined with versatility in function and performance makes it suitable for numerous applications, including homeland security, point-of-care diagnostics, environmental monitoring, forensics, and industrial safety. The combination of plasmonic paper and handheld Raman spectrometers is expected to make SERS a powerful, field-deployable trace detection method. In future work, we will extend the biomimetic SERS substrate approach to other chemical analytes. We also envision a paper-based plasmonic ‘nose’ comprising an array of chemically selective test domains for analyzing complex chemical space.
Srikanth Singamaneni is an assistant professor in the Department of Mechanical Engineering and Materials Science. His research group works on the design, synthesis, and self-assembly of plasmonic nanostructures. He is a recipient of the NSF CAREER award and the Translational New Investigator Award.
Paul M. Pellegrino is chief of the Optics and Photonics Integration Branch in the Sensors and Electron Devices Directorate. His research interests include optics, physics, computational physics, optical transduction (chemical and biological sensing), SERS, quantum control, and photoacoustic spectroscopy.
Wright-Patterson Air Force Base
Rajesh R. Naik is the biological materials research team leader in the Soft Matter Materials Branch, Materials and Manufacturing Directorate. He has published more than 180 peer-reviewed papers on bionanoscience, biointerfacial science, biosensors, and biomaterials.