In a proposed method of sensing small quantities of molecules of interest, surface enhanced Raman scattering (SERS) spectroscopy would be further enhanced by means of intermolecular or supramolecular charge transfer. There is a very large potential market for sensors based on this method for rapid detection of chemical and biological hazards.

A Target Molecule Would Become Bound to a receptor molecule on an SERS substrate. The resulting R-T complex would have a charge-transfer energy band that would coincide with a plasmon resonance, λ1, of the substrate. Optionally, the target molecule could be released through electrochemical reduction.
In SERS, the Raman signals (vibrational spectra) of target molecules become enhanced by factors of the order of 108 when those molecules are in the vicinities of nanostructured substrate surfaces that have been engineered to have plasmon resonances that enhance local electric fields. SERS, as reported in several prior NASA Tech Briefs articles and elsewhere, has remained a research tool and has not yet been developed into a practical technique for sensing of target molecules: this is because the short range (5 to 20 nm) of the field enhancement necessitates engineering of receptor molecules to attract target molecules to the nanostructured substrate surfaces and to enable reliable identification of the target molecules in the presence of interferants.

Intermolecular charge-transfer complexes have been used in fluorescence-, photoluminescence-, and electrochemistry-based techniques for sensing target molecules, but, until now, have not been considered for use in SERS-based sensing. The basic idea of the proposed method is to engineer receptor molecules that would be attached to nanostructured SERS substrates and that would interact with the target molecules to form receptor-target supramolecular charge-transfer complexes wherein the charge transfer could be photoexcited.

As shown schematically in the figure, a SERS substrate would be functionalized with a receptor (R) molecule that has an affinity for a target (T) molecule. The receptor molecule could be designed so that the lowest unoccupied molecular orbital (LUMO) of the target molecule would lie above the highest occupied molecular orbital (HOMO) of the target molecule by an energy difference that would correspond to one of the plasmon resonances of the SERS. Conversely, the plasmon of the SERS substrate could be tailored so that its resonance would lie in the charge-transfer energy band of the R-T complex. In addition to the aforesaid factor-of-108 SERS enhancement, there would be an additional enhancement, by a factor of the order of 103 to 106, contributed by the vibronic energy levels associated with the charge transfer.

With this further enhancement, the detection principle is a form of surface enhanced resonance Raman scattering (SERRS) spectroscopy. The resulting Raman spectrum would consist of a mixture of SERS vibrational peaks from R and T as well more intense SERRS peaks associated with R and T modes that participate in the charge transfer. These strong charge-transfer peaks would enable discrimination of important target molecules from interferants that may also be SERS-active. The sensor/molecule system as described thus far would potentially be reversible in the sense that the R-T interactions could be turned off by applying a bias voltage to electrochemically reduce T to T-. Because T- would no longer have an affinity for R, T could be easily washed away.

This work was done by Eric Wong of Caltech, Amar Flood of the Indiana University Bloomington, and Alfredo Morales of Sandia National Laboratories for NASA’s Jet Propulsion Laboratory.

In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to:

Innovative Technology Assets Management
JPL
Mail Stop 202-233
4800 Oak Grove Drive
Pasadena, CA 91109-8099
(818) 354-2240
E-mail: This email address is being protected from spambots. You need JavaScript enabled to view it.

Refer to NPO-43791, volume and number of this NASA Tech Briefs issue, and the page number.

Topics:
Physical Sciences Research and development Sensors and actuators Spectroscopy
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Enhancing SERS by Means of Supramolecular Charge Transfer

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NASA Tech Briefs Magazine

This article first appeared in the March, 2009 issue of NASA Tech Briefs Magazine (Vol. 33 No. 3).

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Overview

The document discusses advancements in Surface Enhanced Raman Spectroscopy (SERS) and its enhancement through supramolecular charge transfer mechanisms, as developed by researchers at NASA's Jet Propulsion Laboratory. SERS is a powerful analytical technique that enhances the Raman scattering signal of molecules, allowing for sensitive detection and characterization through their vibrational spectra. However, traditional Raman spectroscopy suffers from low sensitivity compared to fluorescence techniques.

The key to SERS lies in the use of nanostructured surfaces that create plasmon resonances, which significantly enhance local electric fields. This enhancement can increase the Raman signal by factors of up to 10^8. The document emphasizes the importance of molecular engineering to attract target analytes to the SERS-active surfaces, which is crucial for effective detection, especially in the presence of interfering substances.

The document also introduces Surface Enhanced Resonance Raman Spectroscopy (SERRS), which combines plasmon resonance with electronic resonance from molecular transitions, leading to even greater signal enhancements (up to 10^14). This technique is particularly useful for molecules with internal resonances or those that can participate in charge-transfer interactions with the metal surface.

Strategies for chemical detection using SERS are discussed, highlighting the use of SERRS-active molecules as tags. While this approach simplifies the requirements for surface coatings, it may reduce selectivity compared to direct vibrational spectra analysis of the analytes.

The document outlines a specific sensing strategy involving supramolecular charge transfer, where a target molecule is attracted to a receptor on the SERS substrate. This interaction creates a charge transfer complex that can couple with the plasmonic field, enhancing the detection signal. The target can be reversibly removed from the substrate through electrochemical reduction, allowing for potential multiplexing capabilities in sensing applications.

Overall, the document presents a comprehensive overview of the techniques and strategies for enhancing SERS, emphasizing the potential for improved sensitivity and selectivity in chemical detection, which could have significant implications for various scientific and industrial applications.