820 Volume 56, Number 7, 2002 APPLIED SPECTROSCOPY0003-7028 / 02 / 5607-0820$2.00 / 0q 2002 Society for Applied Spectroscopy

Remote Detection Using Surface-Enhanced ResonanceRaman Scattering

AILIE MCCABE, W. EWEN SMITH,* GRANT THOMSON, DAVID BATCHELDER,RICHARD LACEY, GEOFFREY ASHCROFT, and BRIAN F. FOULGERDepartment of Pure and Applied Chemistry, University of Strathclyde, 295 Cathedral Street, Glasgow, G1 1XL, UK (A.M., W.E.S.);Department of Physics and Astronomy, Molecular Physics and Instrumentation Group, University of Leeds, Woodhouse Lane,Leeds, LS2 9JT, UK (G.T., D.B.); MOD, P.O. BOX 1666, London, SW1P 1XB, UK (G.A.); and CBD Environmental Services,Building C34, DERA Winfrith, Winfrith Technology Centre, Dorcester, Dorset, DT2 8XJ, UK (B.F.F.); Police Scienti� cDevelopment Branch, Home Of� ce, St. Albans, AL4 9HQ, UK (R.L.)

Surface-enhanced resonance Raman scattering (SERRS) providesintense Raman signals that are shown here to be stable in a targetand to be detectable at least 10 meters from the spectrometer. Theresults indicate that SERRS labeling of objects and their detectionat a distance with a low-power laser is feasible. Rhodamine and adye speci� cally designed to give good surface adhesion, [4(59-azo-benzotriazyl)-3,5-dimethoxyphenylamine] (ABT DMOPA), were ad-sorbed onto silver particles and the particles dispersed in poly(vinylacetate) (PVA) and varnish. SERRS from rhodamine was not de-tected from colloid dispersed either in PVA or varnish, presumablydue to displacement of the dye from the silver surface. ABT DMO-PA gave good SERRS. Maps of the SERRS intensity of � lms indi-cated variability of 10–20% if ultrasound was applied to improvedispersion during mixing. Scattering performance was evaluated us-ing a system with the sample held up to one meter from the probehead. The intensity of the scattering from samples kept in the darkshowed little change over a period of up to one year. However, whenthe samples were left in direct sunlight, the scattering intensitydropped signi� cantly over the same period but could still be deter-mined after eight months. An optical system was designed and con-structed to detect scattering at longer distances. It consisted of aprobe head based on a telephoto or CCTV lens that was � ber-optically coupled to the spectrometer. Effective detection of SERRSwas obtained 10 m from the spectrometer using 3.6 mW of powerand a 20 s accumulation time.

Index Headings: SERRS; Distance detection; Stable SERRS labels;Surface-enhanced resonance Raman scattering.

INTRODUCTION

Raman scattering has advantages as a detection tech-nique in that it gives sharp peaks that enable in situ iden-ti� cation of speci� c molecules.1–3 However, it is an in-herently weak effect and for distance detection4 it re-quires a very powerful laser. Scattering ef� ciency can beenhanced in a number of ways. These include resonanceenhancement,5–7 in which the laser frequency is tuned tothe frequency of a transition of the analyte, and surfaceenhancement,8–13 in which the analyte is adsorbed onto aroughened surface of a suitable metal (usually silver orgold), so that there is an interaction between the metaland the surface plasmon. When the two forms of en-hancement are combined, by adsorbing a dye onto a suit-able surface, surface-enhanced resonance Raman scatter-ing14–19 (SERRS) is obtained. The SERRS effect rivals� uorescence in sensitivity and has been used to detectsingle molecules.20,21 Therefore, it should be possible to

Received 19 October 2001; accepted 25 February 2002.* Author to whom correspondence should be sent.

prepare SERRS substrates that are suitable sampling tar-gets for remote detection.

The advantages of using SERRS for distance detectionare that the scattering is strong, with a detection ef� cien-cy comparable to � uorescence, a much wider range ofchromophores can be used since both � uorophores andnon-� uorophores are effective, and sharp molecularlyspeci� c signals are obtained from each dye.16–19 This lat-ter advantage means that if a number of dyes are ad-sorbed on the same target, either on separate silver par-ticles or in combination on one silver particle, the signalsfrom each dye can be observed in situ without separa-tion.17 This will enable vibrational ‘‘codes’’ to be ‘‘writ-ten’’ into a speci� c detection system.

Many different surfaces have been used to create thesurface enhancement required for SERRS. The mostcommonly used surfaces are roughened silver and goldsince the surface plasmons are in the visible region andcan be tuned by altering surface roughness to match thefrequency of the incident radiation.21–31 In this paper, ag-gregated silver colloid is chosen as the effective surface.The ease with which silver colloid may be dispersed inother materials lends itself to applications in which theSERRS active material is applied to surfaces as a paintor spray.

EXPERIMENTAL

Sample Preparation. The colloid was prepared usinga modi� ed Lee and Meisel method.26,32 Ninety milligramsof silver nitrate was dissolved in 500 mL of distilled wa-ter at a temperature of 40 8C under continuous stirring.The solution was heated to 98 8C and 10 mL of a 1%trisodium citrate solution was added. The temperaturewas maintained at 98 8C for 90 min. The suspension wascooled and the electronic spectrum measured. Colloidwith a maximum absorbance within the range 403–410nm and a full width at half-maximum of 60 nm was usedthroughout this study, as the colloid has been found toprovide a consistent particle size and a small size distri-bution if made by this preparation method.33

Rhodamine 6G was purchased from Sigma, while the4(5 9-azobenzotriazyl)-3,5-dimethoxyphenylamine (ABTDMOPA) was previously prepared in this laboratory. Itwas analyzed for carbon, hydrogen, and nitrogen contentand by NMR. The dye was in an acceptable purityrange.34

Samples were prepared by mixing 5 mL of silver col-

APPLIED SPECTROSCOPY 821

TABLE I. List of sample types.

Sampleprep. Matrix Dispersion method

12345

varnishvarnishvarnishPVAPVA

stirring (a)ultrasonication (c)EtOH re-spin (b)stirring (a)ultrasonication (c)

TABLE II. Lenses used for detection at longer distances.

Camera lensFocal length

(mm) F-number

Aperturediameter

(mm)

Mamiya telephotoCanon telephotoPentax CCTV

30060075

5.64.51.4

53.6133.3

41.25

FIG. 1. Ray trace of detector head set-up.

loid in suspension with 0.75 mL of 1 3 1026 M rhoda-mine 6G (Sigma) or ABT DMOPA in a centrifuge tube.The volume was brought up to 12 mL with distilled waterand centrifuged for 20 min at 3000 rpm. The supernatantwas removed to leave a small silver pellet in the base ofthe tube. The pellet was added to approximately 1 mL ofthe matrix and thoroughly mixed and dried on a glassmicroscope slide. Different methods were used to dis-perse the colloid/dye pellet into the clear, pure color nailvarnish (No. 17, cosmetic range) or PVA matrix. Sampleswere prepared by (a) stirring the colloid/dye pellet intothe matrix, (b) resuspending the colloid/dye pellet in eth-anol before respinning in the centrifuge and stirring intothe matrix, and (c) adding the colloid/dye pellet to thematrix and ultrasonicating the sample for 2 min. The ma-trix was then sprayed or painted onto glass microscopeslides and allowed to dry. Table I lists the different typesof samples that were evaluated in this study.

Sample Characterization. Surface-enhanced reso-nance Raman scattering from the thin � lms was investi-gated using a Raman microscope (Renishaw, System2000) in conjunction with an x,y,z mapping stage withdedicated software (Prior). A maximum excitation powerof 3 mW at 514 nm was focused into a 10-mm spot usingan aplanatic microscope objective (Olympus, MSPLAN,x5). Sample areas of 30 mm (x axis) by 10 mm (y axis)were mapped by collecting spectra of the sample every2 mm (x axis) and 1 mm (y axis) using a 1-s accumu-lation time. The area under the peak at 1366 cm21 in theABT DMOPA spectrum was determined at each pointmapped. The peak areas were then used to generate aresponse map of the sample surfaces.

A series of measurements were also recorded from all

of the sample types stored under three different storageconditions over a period of time. These allowed the com-parison of the time stability of each sample under eachcondition over the initial period of one month.

Laboratory System. A simple remote detection testsystem was used to test and select samples for use in thelonger distance detection system. The collection lens inthe head of a Raman probe (Renishaw, system 100) wasreplaced with one of four uncoated, plano-convex lenseswith a diameter of 24.5 mm and focal lengths of 25, 250,500, and 1000 mm. Experiments were carried out in thedark, in an enclosed box.

Longer Distance Detection. For longer distance de-tection, a probe head was developed using a telephoto orCCTV lens. It was coupled through � ber optics to a Ren-ishaw 100 system using a pair of 0.275 NA, 62.5 mm,multimode optical � bers, (Fiberguide, Anhydroguide UV/Vis 62.5/125). A 532-nm long-pass � lter, (BARR Asso-ciates) was positioned in front of the optical � bers atnormal incidence to attenuate elastically scattered light.The lenses used are listed in Table II. The charge-coupleddevice was the EEV-CCD0206 chip, operated at 270 8C,affording a dark current (N dark), of 0.01 e2 /p/s and a readnoise (N read ) of 5 e2 RMS. The design is such that it couldin the future be coupled to a portable spectrometer (Fig.1).

The SERRS substrate was positioned at a distance ofup to 10 m from the camera lens front surface. A targetwas etched into the microscope slide such that measure-ments could be acquired from approximately the samepoint. Focusing onto the sample was achieved by passinga HeNe (Spectra Physics) 632.8-nm wavelength laserthrough the probe optical � ber in reverse and focusingthe laser to the tightest possible spot at the sample. Two

822 Volume 56, Number 7, 2002

FIG. 2. The structure of the benzotriazole dye 4(5 9-azobenzotriazyl)-3,5-dimethoxyphenylamine (ABT DMOPA).

FIG. 3. ABT DMOPA spectra recorded on the Raman microscope witha 35 objective, a power of 3 mW, 514-nm excitation, and a collectiontime of 1 s. (a) Preparation 2; (b) preparation 5.

frequency-doubled Nd:YAG laser pointers with a wave-length of 532 nm were oriented such that they illuminatedthe substrate within the focused HeNe spot; the totalpower of the two lasers was 3.6 mW. The alignment laserwas disconnected from the probe head, which was thenreconnected to the spectrograph. Two regions of thecharge-coupled device were selected, one to collect thescattering from the sample and one to collect light off-axis to measure the background level.

RESULTS AND DISCUSSION

Two dyes were chosen to provide the target moleculefor detection. Rhodamine 6G is a standard dye used inmany SERRS experiments. Little is understood about thesurface adsorption of this dye and special surface treat-ment is often recommended for SERRS. However, thesignals under the correct conditions are very intense.35,36

To provide a more reliable basis for SERRS, special dyeshave been synthesized in this laboratory.34 These dyes usethe benzotriazole group as part of their structure.

Benzotriazole is an anticorrosion agent for copper andan antitarnish agent for silver. It is believed to work bycomplexing with metal ions on the surface using morethan one nitrogen atom to form a polymer or cluster withthe silver ions on the surface, effectively preventing de-sorption of the dye. The structure of the dye chosen fromthis class of dyes is given in Fig. 2 (ABT DMOPA34).

Intense signals could be obtained from both dyes ad-sorbed on aggregated silver colloid in aqueous suspen-sion. However, when the silver colloid with the dye ad-sorbed was centrifuged and dispersed in PVA or varnish,the rhodamine signals were very weak and readily lostin the � uorescence background while the signals fromABT DMOPA remained strong, and it alone was used insubsequent experiments. The loss of the rhodamine signalmay be due to displacement of the dye from the surfaceduring the preparation process or other effects such assurface deactivation. The result illustrates the need inSERRS for dyes such as the benzotriazole azo dye usedhere to provide reproducible signals in a wide range ofsystems.

Surface-enhanced resonance Raman scattering signalsfrom different points on the surface of replicate samplesof each preparation listed in Table I were recorded from� lms made with the more effectively bonded dye, ABT

DMOPA. The scattering was strong in every case, pre-sumably because of the strong attachment to the surface.Figure 3 shows representative spectra obtained from onepoint on � lms of ABT DMOPA in varnish (preparation2) and PVA (preparation 5) taken with the Renishaw Ra-man system 2000. ABT DMOPA is used exclusively inthe rest of this study.

In order to evaluate the variation in the intensity ofABT DMOPA signals across the surface of the differentPVA and varnish � lms, Raman maps of a number ofsamples were taken. Figure 4 shows the Raman map ofa PVA sample (preparation 4). It indicates that there is alarge variation in intensity across the surface. Visual andmicroscopic examination of the � lm shows that the col-loid is not evenly dispersed. With a varnish sample (prep-aration 1), the variation across the surface is slightlygreater.

Two methods of improving the colloid dispersion wereinvestigated in a further series of experiments. The � rstmethod involves resuspending the original colloid pelletin ethanol, then recentrifuging the sample to form a sec-ond pellet, which is dispersed into the PVA matrix. Im-proved distribution was observed; however, the averageintensity of the SERRS signal had decreased. The secondmethod involved placing the tube containing the colloidand ABT DMOPA mixed with PVA in an ultrasound bathfor 2 min to disperse the colloid before the � lms weredried on a microscope slide. A more even distributionwas observed by visual and microscopic examination ofparticle distribution and by Raman mapping (Fig. 5).Similar results were observed in the varnish � lms. TheRSD for each PVA � lm was calculated using the peakheight of the peak at 1366 cm21. The RSD followingultrasonication was 12% compared to 67% before ultra-sonication.

In order to assess the stability of the SERRS signalsover time, one sample of each of the preparations 1, 2,

APPLIED SPECTROSCOPY 823

FIG. 4. SERRS map for a PVA sample (preparation 4), collection time 1 s, 35 objective, 10% of 3 mW power, and 514-nm excitation. (a)Response bitmap created as the samples are measured shows a square for each measurement taken of the surface. The intensity of the square relatesto the intensity of the SERRS signal obtained. Black is weak and white is intense. (b) 3D SERRS response map created in Grams. The computeraverages the slope between the measured points.

FIG. 5. SERRS results for a PVA sample (preparation 5) with a 2 min exposure in the ultrasound bath, collection time 1 s, 35 objective, 10% of3 mW power, and 514-nm excitation. (a) A response bitmap that shows a square for each measurement taken. The intensity of the square relatesto the intensity of the SERRS signal. Black is weak and white is intense. (b) A SERRS response map created in Grams from the original data.

4, and 5 were stored under three different conditions.These were (1) in the dark protected from the atmosphereinside a closed microscope storage box, (2) in sunlightexposed to the atmosphere but protected from the weatherthrough glass, and (3) in sunlight exposed to the atmo-sphere and the weather.

The stability was assessed over one year. When storedunder condition (1) samples were stable for the wholeperiod. Signal intensity remained relatively consistent upto one year after preparation. When stored under condi-tion (2) samples showed some loss of signal over time;however, signals were still identi� able after eight months.When stored under condition (3) the samples in each ma-trix responded differently. The varnish samples (prepa-rations 1 and 2) showed some loss in signal over theinitial time period and the signals were last successfullyrecorded 3 weeks after preparation. The PVA samples(preparations 4 and 5) lost signal with time more quicklyand were last recorded after 2 weeks.

Remote Detection. An optical bench for SERRS de-tection was set up in the laboratory and the SERRS re-sponses at different distances from all the sample typeswere recorded. Since scattering forms a wide cone oflight, the further the sample is from the instrument, thesmaller the signal expected. All samples gave a similarand reproducible distance dependence with a reduction insignal consistent with that found in the longer distanceexperiments. From this study and the time stability ex-periments, a PVA sample prepared with ultrasonication

was chosen to develop the collection optics for the newlonger distance modi� ed system.

Longer Distance Detection. For practical use, greaterdistances would be of considerable bene� t, as would amore � exible collection system that could be coupled toa portable spectrometer in due course. A simple effectivesystem was to use either a CCTV lens or a telephoto lens� ber-optically coupled to the Renishaw system 100 spec-trometer, (referred to as the modi� ed system). Powers ofup to 3.6 mW were used, although from the point of viewof health and safety, powers of less than 1 mW will berequired in practice.

An estimation of the signal levels achievable using theproposed system is obtained by considering the differ-ence in the system response function between the originalunmodi� ed Renishaw system 100 (referred to below asthe system 100) and the modi� ed system. The major dif-ferences arise from a reduction in the numerical apertureof the collection lens (NA), the transmission character-istics of the � lters used to reject elastically scattered light(t), the integration time allowed (tint), and the laser powerat the sample. A conversion factor (g), can be consideredas the product of the ratios of each of these considerationsand is shown in Eq. 1. The subscripts ‘R100’ and ‘Ms’refer to the system 100 spectrometer and the modi� edsystem respectively.

2NA f t tR100 R100 R100 int R100g 5 · · · (1)1 2 1 2 1 2 1 2NA f t tMS Ms Ms int Ms

824 Volume 56, Number 7, 2002

FIG. 6. Graphical determination of maximum standoff distance. (a) 75mm, f /1.4 CCTV Lens (Moonlight); (b) 75 mm, f /1.4 CCTV Lens(Absolute Limit); (c) 600 mm, f /4.5 Telephoto Lens (Moonlight); and(d ) 600 mm, f /4.5 Telephoto Lens.

FIG. 7. Graph showing effect on spectral intensities as the distance isincreased, using the modi� ed collection optics on a Renishaw System100 with an excitation of 3.6 mW at 532 nm and a collection time of20 s. The intensity of signals are decreasing from (a) 2 m, (b) 3 m, (c)4 m, (d ) 6 m, and (e) 10 m.

FIG. 8. Comparison of the effect on signal-to-noise ratio of distanceobserved experimentally and the theoretical prediction. The Renishawsystem 100 with modi� ed collection optics was used with an excitationof 3.6 mW at 532 nm and a collection time of 20 s. (a) Experimentalresult with the 75 mm, f /4.5 Pentax CCTV lens. (b) Modeled relation-ship using the allometric function.

The proportion of light gathered by the optical � berused to connect the collection lens to the spectrographmay be approximated by computing the proportion of thesample image, formed by the CCTV lens, collected bythe optical � ber of core radius (r� ber). The point spreadfunction increases the size of the image and reduces theeffective collection ef� ciency. The rejection of the elas-tically scattered light was achieved using a proprietarydielectric thin � lm long-pass � lter (Barr Associates). Thecollection lens was used to focus inelastically scatteredlight through the � lter onto the optical � ber. This enablesthe full aperture of the collecting lens to be spectrally� ltered using a small � lter. The spectral characteristics ofthe long-pass � lter are polarization- and angle-dependent.The transmissivity in the spectral region of interest toSERRS approaches 98% while maintaining an opticaldensity blocking of at least 4.0 at 200 cm21 at the exci-tation wavelength. The limit of detection is de� ned as thepoint at which the signal-to-noise ratio reaches 3:1, al-though successful detection may be achieved below thisby using more aggressive spectral processing algorithms.The treatment of signal-to-noise ratio in this model fol-lows the work of Puppels and Greve.37–39

In the modi� ed arrangement, the microscope objectivelens limits the numerical aperture of collection, NAR100,to 0.1. The effective scaling factor (g) using Eq. 1 wascalculated to be 50.7 using the collection parameters de-scribed in Fig. 1.

Since the complete spectrum is required in order tomake use of the molecularly speci� c advantage ofSERRS, the signal-to-noise ratio was calculated using theABT DMOPA spectral feature at 1445 cm21 as this hasthe narrowest bandwidth of the weaker spectral features.The background was subtracted before use. Assumingthat the noise associated with each binned-pixel readoutfrom the CCD is statistically independent of the shot-noise, then the total noise (N tot) associated with any in-dividual spectral feature is their sum in quadrature. Thesignal-to-noise ratio of the 1445 cm21 peak was estimatedto be 11.4:1, indicating that collection at more than 5 mis feasible. The graph in Fig. 6 shows the calculated sig-

nal-to-noise ratio as a function of distance (d ) for aCCTV lens and a telephoto lens.

The sample was positioned at distances of 2, 3, 4, 6,and 10 m from the collection lens and spectra were re-corded over 20 s. The results are shown in Fig. 7. Thesignal-to-noise ratio of each spectrum in Fig. 7 was plot-ted against distance. The agreement between experimentand prediction is good, as can be seen for the CCTV lensin Fig. 8. To estimate the effect of ambient light level onthe signal-to-noise ratio, the ratio for the 75-mm CCTVlens was curve-� tted using the function SNR(d ) 5 x ·d 2 y, where x is the signal-to-noise ratio, y is the decayconstant of the curve, and d is the distance from the sam-ple.

The reduction in signal-to-noise ratio at increasing lev-els of standoff was found to fall according to x21.39, whichis consistent with the decay limits identi� ed theoretically

APPLIED SPECTROSCOPY 825

FIG. 9. SERRS spectrum acquired at a distance of 10 m. Excitation of 3.6 mW at 532 nm, collection time of 1 s, using a Renishaw System 100with modi� ed collection optics. (a) 600 mm, f /4.5 Canon telephoto lens; (b) 75 mm, f /1.5 Pentax CCTV lens; and (c) 300 mm, f /5.6 M.

of d 21.79 and d 21.00. The backgrounds observed at in-creased remote distances are not constant due to nonuni-form illumination in the laboratory over the distanceused. The log–log plot of signal-to-noise ratio shows agradient of 0.52 1/20.03, which is close to the theoret-ical shot-noise limited value of 0.5.

The modeling results suggest that CCTV lenses are notideal for this application since their low cost comes atthe expense of imaging quality. They are typically de-signed for imaging using detector pixel-limited resolu-tion, and so have a point spread function of approxi-mately 30 mm. However, they are a relatively inexpensiveand readily available source of complex lenses, and onewas evaluated in the � nal experiment.

The sample was repositioned at a distance of 10 mfrom the collection lens and the spectra recorded usingeach of the lenses in Table II using a 1 s integration time.The results show that SERRS is effective for distancedetection with a low-power laser and a short 1 s accu-mulation time (Fig. 9).

CONCLUSION

It is possible to use surface-enhanced resonance Ra-man scattering (SERRS) for remote detection of speci� -cally designed substrates. Rhodamine 6G did not adherestrongly to the colloid surfaces and was displaced duringsample preparation. It is not suitable for the samplesmade in this study. ABT DMOPA is a dye specially de-signed for SERRS and was strongly adsorbed onto silverparticles, allowing the particles to be dispersed success-fully in PVA and varnish. The SERRS intensity distri-bution across the surface is more homogeneous when anultrasonic bath treatment is incorporated into the prepa-ration procedure. The sample substrates are stable withtime for between two weeks and one year, dependant onthe storage conditions. Best results are gained from sam-ples stored in the dark and protected from the atmo-sphere.

Initial experiments with a Renishaw Raman system100 produced good SERRS up to 1 m from the collectionlens. A theoretical model of the designed system wasused to manipulate an example SERRS spectrum and to

estimate the expected signal intensities at a distance of10 m.

Detection up to 10 m was successfully obtained usingthe modi� ed system. The advantage of distance detectionby SERRS is that it offers a sensitivity to rival � uores-cence, and sharp molecularly speci� c signals are ob-tained. Consequently a number of different dyes can bediscriminated without separation. The design of the col-lection optics is such that it could be attached to a smallportable instrument.

ACKNOWLEDGMENTS

We would like to thank the M.O.D. for funding.

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