Paper test strips for explosives detection

There is a growing demand for trace chemical detection outside of the lab for applications including identification of toxins in water sources, pesticides on produce, and explosives on surfaces. For example, as the threat of urban terrorist attacks worsens, the need for rapid detection in the field increases. Also, as we acquire more knowledge about the danger of certain toxins and carcinogens, the demand to detect them in food, water, and the environment grows. The most common analytical technique used to identify chemicals combines liquid chromatography (LC) and mass spectrometry (MS). LC-MS shows excellent detection performance, but the equipment is bulky, expensive, and accordingly inappropriate for use in the field.

In the past three years, paper-based analytical systems have emerged as an exciting possibility for ultra-low-cost analysis.^{1, 2} A variety of transduction techniques have been reported, including colorimetric,^{3, 4} chemiluminescent,^5,6 and electrochemical detection.^{7, 8} However, despite the rapid progress in this field, significant advancements are still necessary to achieve highly sensitive detection of a broad range of chemical targets. Surface-enhanced Raman spectroscopy (SERS) has the desired sensitivity, yet bulky equipment and costly single-use substrates have confined it to research laboratories. We have taken a step toward detection in the field by developing ultra-low-cost SERS paper devices that could in theory be used to detect any molecules, from pesticides to explosives.

Figure 1. (A) Sensing spots of silver nanoparticles (Ag NPs) printed on paper for surface-enhanced Raman spectroscopy (SERS). (B) Scanning electron micrograph of Ag NPs inkjet-printed onto cellulose paper. (C) Raman spectrum of 100 attomoles of Rhodamine 6G (R6G) on paper detected using a hand-held spectrometer.

In Raman spectroscopy, photons from a laser source are scattered at frequencies corresponding to the vibrational energies within molecules. This scattering of photons is a rare event, making standard Raman spectroscopy unsuitable for trace chemical detection. In SERS, the Raman signal is boosted significantly by metal nanostructures adsorbed to a surface.^9–15 SERS is sensitive enough to identify single molecules.^15–18

Recently, our group reported the fabrication of SERS substrates on cellulose paper using inkjet printing.¹⁹ To create the SERS-active regions, we used an inexpensive ($60) consumer printer to print 1mm spots of silver colloid ink onto cellulose paper: see Figure 1(A) and (B). To prevent the sample droplets from spreading, we first made the paper surface hydrophobic by printing a solution of hexadecenyl succinic anhydride, a common paper sizing agent.

To characterize the performance of the paper-based SERS-active arrays, we used Rhodamine 6G (R6G) as a model analyte. We pipetted 1μL droplets of R6G in water onto the SERS-active spots and analyzed them using a Raman microscope. We were able to detect the R6G spectral features after applying as few as 10 femtomoles of R6G to the SERS-active spots. To demonstrate the applicability of the inkjet-printed SERS detectors for field use, we used a fiber-optic probe in combination with a diode laser and a portable Raman spectrometer. The fiber-optic probe contains two cables, one to direct light from the laser to the paper and the other to collect the Raman-scattered photons and deliver them to the spectrometer. With this portable measurement apparatus and further improvements to our inkjet printing process, we were able to detect 100 attomoles of R6G: see Figure 1(C).

Figure 2. (A) SERS paper dipstick with printed Ag NP spot. (B) Raman signals recorded for a 1μL sample droplet of 5ng/mL R6G in methanol spotted onto a paper-based SERS sensor and for 250μL of 50pg/mL R6G detected using the SERS dipstick.

While the results in Figure 1 show that inkjet-printed SERS substrates can serve as sensitive chemical detectors, the need to apply microdroplets of samples may not be optimal for field-based applications. To make a more practical analytical device, we have developed a paper dipstick that makes use of the fluidic properties of cellulose paper to concentrate analyte molecules from a relatively large sample volume into a small SERS-active spot. We put the SERS dipstick in a reservoir to collect the sample and wick analyte molecules toward the inkjet-printed detection zone: see Figure 2(A). We then add a solvent to the reservoir to wick the molecules further up to the tip, where they can be detected. In tests, the SERS signal recorded at the tip of the dipstick was two orders of magnitude stronger than that recorded with a droplet pipetted onto paper: see Figure 2(B).

These ultra-low-cost (ca. $0.03) paper-based SERS devices represent a new paradigm in analytics. The devices are very easy to make and use. The SERS dipsticks can simply be placed in a liquid sample, in contrast to careful pipetting or to the complex equipment used for microfluidic devices. Alternatively, we can simply wipe the dipstick over the surface being tested to collect trace chemicals.

We will soon begin testing our SERS-active paper for explosives detection. As our paper sensors become more sophisticated, we can also imagine using them as wound swabs to look for infection. We expect that the commercial introduction of paper-based SERS substrates will finally enable field-deployable SERS for applications in trace chemical detection.