Surface Enhanced Raman Spectroscopy of Individual Suspended

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Surface Enhanced Raman Spectroscopy of Individual Aerosol Particles Vasanthi Sivaprakasam, Matthew Hart, and Jay D. Eversole J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b05310 • Publication Date (Web): 14 Sep 2017 Downloaded from http://pubs.acs.org on September 19, 2017

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The Journal of Physical Chemistry

Surface Enhanced Raman Spectroscopy of Individual Suspended Aerosol Particles Vasanthi Sivaprakasam*, Matthew B. Hart and Jay D. Eversole Naval Research Laboratory, 4555 Overlook Avenue SW, Washington, D.C. 20375, USA

ABSTRACT We report observation and measurement of Surface Enhanced Raman Spectroscopy (SERS) signatures from individual suspended aerosol particles. To our knowledge, this is the first report of SERS from single suspended particles formed from droplets containing analyte molecules and metallic nanoparticles (MNPs). We describe our experimental setup to generate, charge and trap micro-droplets, and an associated spectroscopic measurement arrangement configured to easily switch between trapped, suspended particles and bulk liquid samples of the same materials. Trapped droplets quickly dry to form micron-sized particles mainly composed of inert material, NaCl (>99%) with trace amounts of analyte molecules and MNPs. As an initial investigation and demonstration of our method, Surface Enhanced Resonance Raman Spectroscopy (SERRS) is conducted on 3 to 5 µm diameter suspended solid particles containing R6G dye molecules and silver nanoparticles (NPs). SERRS signal intensities are linear with particle dye concentrations up to a saturation point. A detection limit of 105 molecules (80 attograms in mass, which would

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be a 50 nm equivalent sphere) is established in particles containing approximately 300 Ag NPs. Comparison with an external standard with a known spontaneous Raman cross-section establishes a SERRS analytical enhancement factor of 105 for these particles. Comparable SERRS enhancement factors are obtained from similar samples as bulk suspensions, and SERRS spectra show good agreement between solid aerosol particles and bulk liquid suspensions. We plan to extend our study to quantify SERS response from atmospheric and threat aerosols to evaluate the feasibility of implementing this technique for ambient aerosol evaluations.

1. INTRODUCTION Spontaneous Raman spectroscopy is a widely used non-contact technique that measures the vibrational and rotational modes of molecules for chemical identification of materials with wideranging applications such as pharmaceutical1 and food industries2, artifact authentication3 and obtaining bio-molecular functional information in medical research4. There is great interest in applying Raman spectroscopy to probe the atmosphere for ambient5, 6 and man-made aerosols7, and within the defense community to detect chemical8 and biological9 agents released as aerosols.

Traditionally, due to typically low spontaneous Raman cross-section values,

measurements require longer acquisition times and/or higher sample concentrations; relegating it to be mostly a laboratory analysis tool. However, recent advances in detection technology have enabled researchers to obtain spontaneous Raman spectral signatures from single particles levitated in either an optical or electromagnetic trap, or from ensembles of flowing aerosol particles10-14.

Raman spectra of trapped droplets in an electrodynamic balance has been

measured as a function of the size of the evaporating droplet to establish volume dependence10. In a similar approach, Lee and Chan report on single particle spontaneous Raman investigation of organic aerosols held in an electrodynamic trap to explore long-term effects of ozone on oleic


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acid particles under controlled temperature and humidity5.

A larger category of aerosol

investigations has been reported using Raman methods on aerosol particles as collected samples, and a selection of related papers are briefly summarized here. Steer et al. report on Raman measurements of size selected and collected aerosols of ZnO7. They are able to chemically discriminate samples using quantitative measurements. Doughty and Hill use a semi-continuous sampling instrument to probe the Raman signature of collected ambient aerosols on a substrate15. They determined empirical spectral clustering among ambient aerosols into: organic, inorganic, soot and humic-like classes.

Despite these significant advances, to date, relatively few

researchers have explored the possibility of obtaining Raman spectroscopic signature from individual aerosol particles in real-time, although techniques to improve the sensitivity of such measurements continue to be investigated8. Extrapolations from prior studies and reported Raman cross-sections for chemicals of interest16 indicate that enhancements of about 103 over spontaneous Raman cross sections would be required to achieve detection from micron-sized, individual chemical agent aerosols flowing past an optical interrogation system. In this work, we demonstrate a method that could potentially improve the ability to detect aerosol particles in situ using Surface Enhanced Raman Spectroscopy (SERS). Development of SERS methods has been an active field for several decades and has been applied to topics ranging from bio-medical applications to gas and explosives detection17-20. SERS is a potentially very sensitive technique capable of single-molecule detection21-23. Various configurations have been previously explored that place target compounds onto specialized metallic substrates or held in suspension with colloids of metallic nanoparticles (MNPs). These techniques have also been applied for enhancing both Raman and fluorescence signatures in biological and chemical defense applications24-27 on collected surface samples as well as



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including collected biological aerosols in a liquid suspension of MNPs18. SERS is a promising technique for studying composition of atmospheric aerosols and also reactions leading to formation of secondary organic aerosols. A recent study has measured SERS spectra of micronsized atmospheric secondary organic aerosols that were collected onto a substrate coated with Ag NPs28. SERS was observed from attograms to femtograms of species present in the aerosol at modes corresponding to SO42-, NO3-, and C-H, O-H stretching regions by Raman microscopy28. In another study, measurements were made using tip-enhanced Raman spectra (TERS) from nanometer sized particles that were generated in the laboratory to simulate atmospheric aerosols29. Nanoparticles formed above a salt solution pool inside an aerosol smog chamber were impacted on microscope cover slips and TERS was performed to identify various sulfates, nitrates and carbonates in the spectral range from 400 to 1800 cm-1. In the study presented here, we apply SERS to interrogate single suspended, micron-sized composite aerosol particles containing MNPs and analytes of interest. There are two distinct advantages of conducting SERS measurement on suspended aerosols, compared to aerosols collected on substrate. First is the possibility that this technique could lead to in situ measurement, where the sampled aerosols from the atmosphere are coated with MNPs inline prior to downstream Raman detection. Second is the possibility of higher MNP analyte interaction due to the possible surface or volume (when aerosol is semi-solid) coverage of the aerosol with MNPs (not just the relatively small contact area between an aerosol particle and a substrate). To validate our technique, we compare our SERS measurements from composite individual aerosols, with bulk solutions prepared with the same analytes and colloids of MNPs. Spontaneous Raman spectra of aerosol and bulk samples of the same analyte materials are measured when possible, for comparison of their spectral features and for quantification of enhancement factors.


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2. EXPERIEMNTAL SETUP An experimental setup was developed to easily measure Raman spectra from either suspended single aerosol particles in a linear electrodynamic quadrupole (LEQ) trap or from bulk samples in a cuvette as shown in Fig. 1. A 532 nm wavelength CW laser (Coherent Inc., Model Verdi5W) is used as the light source to generate the Raman spectra and a 45° flip-mirror diverts the beam to either the bulk or aerosol sample. A 657 nm wavelength diode laser is collinear with the 532 nm laser, and is aligned along the axis of the LEQ. The red laser is used both to monitor the position of the trapped particles by imaging their elastically scattered signal onto a CCD camera, and to infer the size of liquid particles by comparing the integrated scatter with Mie theory. Both the elastically scattered light used for sizing, and the Raman spectrum are collected and collimated using the same f/1.2 lens along an orthogonal direction to the imaging camera as shown in Fig.1. The collimated light is split into two branches using a 654 nm short-pass dichroic beam splitter (BS) (Semrock Inc., Part# FF654-SDi01). This optic reflects the 657 nm elastically scattered light into a photo-diode (PD), which is recorded as a function of time. The transmitted light is then passed through a 650 nm short-pass filter (Thorlabs Inc., Part# FESH0650) and a 532 nm ultra-steep long-pass edge filter (Semrock Inc., Part# LP03-532Ru-25) to minimize the transmission of the remaining 657 nm and 532 nm elastic laser scattering signals respectively. The light is then focused onto the spectrometer slit (nominally set to 100 µm width) using an f/4 lens that matches the 320 mm focal length spectrometer (Princeton Instruments, Model# 320i). The spectrum is recorded by a CCD detector of 2048 x 512 format with pixel dimensions of 10 µm x 10 µm (Princeton Instruments, Model# PIXIS 2KBUV).



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Similar to diverting the incident beam, another 45° flip-mirror is employed to switch between collecting Raman spectra from bulk or aerosol samples.

Figure 1. Experimental layout of the Raman spectroscopy setup. Spontaneous Raman and SERS spectra can be measured from bulk samples in cuvette or as aerosols held in a LEQ trap. The top view is shown on the left – a 532 nm laser is used as excitation source and is diverted to bulk or aerosol measurement setup by employing a flip-mirror. Similarly, collected light is dispersed and detected by the same spectrometer and CCD detector for both types of samples. The side view of the aerosol chamber is shown on the right enclosing the LEQ rods and collinear 532 nm Raman laser and a 657 nm laser beam used for particle sizing and positioning. A droplet generator with a conductive reservoir and capillary tube is shown on top of the aerosol chamber. An LEQ trap utilizes the electrodynamic quadrupole fields created by four parallel, squarely arranged conductive rods with an alternating voltage applied to pairs of diametrically opposing


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rods. The time-varying field constrains charged particles to an axis parallel to the rods and located in the center of the square geometry defined by the rods as shown in Fig. 1. Therefore, particles are constrained in two dimensions, located along this axis, and absent other forces, can freely move along it. With the system aligned vertically, positional control of particles within the LEQ is accomplished by balancing the forces of gravity and a downward air flow against an electrostatic balancing field at the bottom of the chamber. A small pump is used to pull HEPA filtered, room air through the LEQ at rates ranging from 0.05 L/min to 0.3 L/min. The balancing field is created by applying a potential of the same polarity as the particle charge to the metal air flow outlet tube at the bottom of the cylindrical enclosure. By monitoring the position of the aerosol using the imaging camera, we provide a real-time feedback to adjust the DC potential of this tube, maintaining the particle at a fixed position in the center of the focal volume of the collection lens. Using this active control, particles are held stationary to within a few percent of their diameters. By controlling the frequency and amplitude of the applied AC voltage to the quadrupole, particles and droplets over the size range of 0.5 to 100 µm have been successfully trapped for hours or even days. Frequencies range from 50 to 3000 Hz, with smaller particles generally requiring higher frequencies for stabilization. Additional details regarding the design and operation of the LEQ trap are provided elsewhere30. Droplets are charged and introduced into the LEQ by means of a nominally 100 µm ID glass capillary tube originating from the bottom of a conductive reservoir. The capillary is located directly above, and centered on, the LEQ axis as shown in Fig. 1 (Side View). High voltage between 1 kV and 3 kV is applied to the reservoir, which forces the liquid to break off into droplets as it exits the capillary and becomes entrained along the LEQ axis. This method of droplet generation is similar to electrospray, but the applied voltage is lower such that the



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droplets formed initially carry a charge just below the Rayleigh limit31,

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, which avoids

formation of a Taylor cone, and results in droplet diameters ranging from about 30 µm (14 pL) to 100 µm (535 pL). The diameter of the generated droplets can be coarsely controlled by varying the diameter of the capillary and the applied voltage on the reservoir. If the droplets are mainly composed of water or other volatile liquids, as is the case in our current study, they evaporate relatively quickly (< 1 s) to form solid (nearly dry) micron-sized particles depending on the suspended or soluble residue. Besides the constant filtered room airflow, at a nominal rate of 0.2 L/min, no other environmental controls are implemented to control the temperature or humidity in the aerosol chamber. Nominally the laboratory temperature is maintained at 65°F to 75°F and humidity ranged from 30% to 80%. Most of the spectra reported in this paper are obtained by using laser powers between 10 mW and 500 mW, with a beam waist of about 80 µm which result in laser fluences of 0.2 kW/cm2 to 10 kW/cm2. Acquisition times ranged from 10 to 300 seconds. Spectra are vertically binned across 165 pixel rows of the CCD camera, corresponding to accumulating signal from 0.5 mm (for system magnification of about 3.3) translation along the laser beam direction in the LEQ for aerosol acquisition, and along the beam height direction in the cuvette for the bulk sample acquisition.

All the SERS measurements are conducted by preparing samples using

commercially available colloids of Ag from BBI Solutions. We note that many researchers prepare metal sols in the laboratory, following a previously published prescription (such as that of Lee and Meisel33). We chose to use a commercially available product to ensure repeatability and for ease of use. 40 nm and 80 nm spherical Ag sols were used in our experiments with less than 12% coefficient of variation in size (BBI Solutions). SERS intensity has been shown to vary as a function of time and is also dependent on the concentration and type of electrolyte used


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to activate the sols34, 35. Common practice has been to prepare the sample and let it sit for a few hours before performing SERS measurements22, 36. Since our future goal is to implement this technique for real-time measurements, our samples are prepared and spectra were measured as soon as possible, which is within a time frame of 10 minutes for bulk samples and within 30 minutes for the aerosol samples.

3. RESULTS AND DISCUSSION 3.1. Spontaneous Raman from a Droplet of Neat Liquid A spontaneous Raman spectrum measured from a levitated droplet of dibutyl sebacate (DBS), C18H34O4 is shown in Fig. 2. The spectrum is measured using a laser power of 500 mW (fluence of 10 kW/cm2) with 300 seconds of acquisition time. The long exposure required to measure the spectrum demonstrates the need for enhanced Raman methods to detect micron-sized aerosols for materials with weak Raman cross-sections. The peaks corresponding to DBS are marked in the figure. The large features in the 2800 to 2950 cm-1 region correspond to the C-H stretch vibrations, while peaks in between 1300 and 1750 cm-1 correspond to the C=O and C-O-C vibrational bands. DBS has a low vapor pressure, and does not noticeably change size due to evaporation over the measurement period. The droplet is trapped in ambient air and the Raman signature of surrounding O2 (1554 cm-1) and N2 (2331 cm-1) can also be identified in the spectrum in Fig. 2, and used for calibration as done by Aggarwal et. al.8. The residual 532 nm elastic scattering signal that is transmitted through the OD 7 long-pass filter and a long tail at the start of the pass band is observed in the 0 to 140 cm-1 region. Direct imaging is accomplished by using back lighting from a 631 nm LED which is used to estimate the size of particles/droplets



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that are larger than 5 µm in diameter. The uncertainty in sizing is determined by the optical resolution of the imaging system, and is about 1 micron, corresponding to 20% for 5 µm particles, or less than 2% for 50 µm particles. An image of the DBS droplet is shown in the inset of Fig. 2 and its diameter is estimated to be 37 ± 0.5 µm. Higher precision sizing (on the order of a nm) of evaporating droplets of neat materials can be achieved by comparing the temporal profile of the 657 nm elastic scattering signal to Mie theory, an example of such measurement conducted on glycerol droplets using similar experimental setup is discussed elsewhere30.

Figure 2. Spontaneous Raman spectrum of a levitated Dibutyl Sebacate (DBS) droplet. The Raman peaks of DBS are highlighted in the figure along with the Raman peaks of O2 and N2 of air. The spectrum was measured at 10 kW/cm2 laser fluence and 300 s exposure time. A direct image (inset) was used to size the droplet.

3.2. SERRS of R6G Bulk Sample


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We chose Rhodamine 590 chloride (R6G) to investigate since it is a well characterized SERS material with a relatively large SERS cross-section. The interrogation wavelength of 532 nm coincides with the absorption peak of the dye molecules, and hence our signal includes resonance Raman effects. A direct measurement of spontaneous resonance Raman spectra is not possible with our approach due to interference from fluorescence of the dye molecules. However, resonance Raman cross-sections of this compound have been obtained using alternate measurement techniques, such as femtosecond stimulated Raman spectroscopy37 or a polarization difference technique38. The Raman measurements we obtained result from two independent mechanisms: surface enhancement due to the presence of Ag NPs, and resonance enhancement, which are referred to in combination as Surface Enhanced Resonance Raman Spectroscopy (SERRS). We obtained these spectral signatures both as bulk samples (solutions) and as aerosols. For both of these measurement configurations, an external standard with a known cross-section, ethyl cinnamate, was used to calibrate our intensity measurement so that we could determine the combined SERRS cross-section and ultimately an analytical enhancement factor for R6G. For the bulk measurements, 10-7 M to 10-10 M R6G solutions in water are made and combined with 2 x 10-12 M 80 nm Ag NPs in water and 10-2 M NaCl solution to activate the MNPs39. The SERRS spectrum of 10-8 M concentration of R6G is shown in Fig. 3 along with the spontaneous Raman spectra of neat ethyl cinnamate.

Both spectra are recorded under

identical optical conditions using 100 mW of laser power (2kW/cm2 fluence) with 10 s exposure time. The SERRS spectrum of R6G features sharp Raman peaks riding on top of the broad fluorescence emission of the dye molecules. The fluorescence intensity of the dye is quenched by at least two orders of magnitude compared with fluorescence from solution of only R6G dye



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molecules of the same concentration.

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This observation is similar to observation by other

reports37, and it is the combination of fluorescence quenching phenomenon together with Raman enhancement that allows the relatively weak SERRS signals to be observed. The fluorescent emission of a molecule can be affected by the presence of MNPs through two competing processes that are dependent on the spacing between the MNPs and analyte molecules; a field enhancement that results in florescence enhancement, or a non-radiative damping (quenching) of fluorescent emission due to energy transfer from the fluorescing molecule to the MNPs40-42. The latter is observed in our case.

Figure 3. SERRS spectrum of 10-8 M R6G suspension with 2 x 10-12 M 80 nm Ag NPs and 10-2 M NaCl bulk sample. The broad background is the fluorescence spectrum of R6G along with sharper peaks of Raman emission. Spontaneous Raman spectrum of neat ethyl cinnamate used as an external standard is also plotted, scaled by 0.5 to fit on the same scale. Both spectra were measured at 2 kW/cm2 laser fluence and 10 s exposure time.


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We note that no SERRS signal is observed until the addition of NaCl. We believe that this is due to the relatively low concentrations of analytes and MNPs, and thus, large mean spacing between MNPs and the analyte (R6G) molecules. The activation of Ag NPs by the addition of NaCl has been previously observed to facilitate SERS by either of two processes: (a) by causing MNP aggregation, or (b) due to formation of surface complexes of analyte molecules and Cl anions to Ag NPs39, 43. In our case, the extinction spectrum of the Ag NPs is not modified by the addition of NaCl, (i.e., the extinction spectrum does not red-shift which would indicate formation of aggregates) therefore aggregation of MNPs can be ruled out. This suggests that formation of surface complexes, which increases the probability of analytes in close proximity to the MNPs, is perhaps is the mechanism leading to SERS in our case. To quantify the SERRS response, the area under the curve after fluorescence background subtraction, is integrated at several of the Raman peaks centered at: 617, 777, 1367, 1514 and 1657 cm-1. For fluorescence, we plot the peak intensity for a 50 cm-1 band at its maximum value (600 cm-1). These results are plotted as a function of dye concentration in a log-log plot in Fig. 4. A dotted line with a slope of unity (denoting a linear relationship between signal and the concentration of the analyte) is plotted for visual aide for both the florescence and the SERRS signals. The SERRS response is near linear as a function of R6G concentration up to about 10-8 M where it begins to saturate. The hydrodynamic radius of a R6G molecule is 0.6 nm44, implying that a maximum of about 1.8 x104 R6G molecules can be in close packing (single layer) contact with each MNP. Given the concentration of 80 nm Ag NPs is 2 x 10-12 M, and assuming minimal agglomeration as discussed above, the upper bound concentration of R6G molecules in close contact, forming a single layer is 3 x 10-8 M. This is in good agreement with the saturation observed in Fig. 4 for concentrations of 10-8 M and higher.


This agreement between the


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observed saturation region and the concentration of close contact molecules for R6G implies that almost all the molecules present in the sample are contributing to SERRS up to the observed saturation limit. Increasing the concentration of the dye further does not result in additional SERRS signal. Under this assumption, one would also expect the fluorescence signal to increase proportionally to the concentration of free dye molecules past the saturation concentration, but this is not observed in our case. Experiments to quantify the concentration of the analyte in the supernatant after centrifuging the sample to separate sols with bound analyte (since surface complex formation is likely in our case), similar to that conducted by Hildebrandt and Stockburger36 may shed some light on this discrepancy and will be pursued in a later study.


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Figure 4. SERRS and fluorescence signal of R6G bulk sample measured as a function of R6G concentration. Dotted lines with unity slope are shown on the plot for visual comparison. The response is linear (slope of 1) for lower concentrations, and exhibit a predicted saturation at the higher concentrations. A limit of R6G detection for given experimental conditions is 10-10 M. A total enhanced Raman cross-section can be computed by comparing our observed SERRS signal of R6G to the Raman signature of an external standard for which the spontaneous Raman cross-section and molecular concentration are both known. We used a previously measured cross-section value of 3.10 ± 0.19 x 10-28 cm2 for ethyl cinnamate determined at its vibrational band at 1638 cm-1 with an excitation wavelength of 785nm45. Therefore, a factor of (785/532)4



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must also be incorporated to compensate for the excitation wavelength difference. This scaled cross-section is multiplied by the ratio of the measured intensities at 1504 cm-1 for R6G and 1638 cm-1 for ethyl cinnamate from Fig. 3 (normalized to their respective molecular densities), which results in a SERRS R6G cross-section of 3.7 ± 1.5 x 10-20 cm2, under our experimental conditions.

Our experimental R6G data results provide good reproducibility, with a 40%

standard deviation for the SERRS cross section observed from 5 SERRS measurements including two different batches of MNPs. Furthermore, we postulate that our total enhanced R6G cross section is the product of two enhancement mechanisms: a SERS enhancement due to the MNPs, and a resonance Raman enhancement. The R6G resonance Raman cross-section has been independently reported by Shim et al.37 for the same excitation of 532 nm and the same 1504 cm-1 band to be: 2.2 x 10-23 cm2. Dividing our experimental SERRS cross-section by this (resonance only) cross-section, yields a SERS analytical enhancement factor of 1.7 x 103. Additionally, we can use the spontaneous Raman cross-section reported by Rue et al.46 of 2.9 x 10-26 cm2 at 633 nm for the same 1504 cm-1 R6G band, and scale that value to compensate for the excitation wavelength difference ((633/532)4), to obtain a total SERRS analytical enhancement factor of 6.4 x 105. Using these two enhancement factors, we infer a resonance enhancement contribution of a factor of 400. In most SERS measurements, the analytical enhancement factor (enhancement computed by the ratio of measured SERS signal to spontaneous Raman signal) is typically orders of magnitude weaker than the true enhancement factor (enhancement factor obtained after compensating for the number of molecules contributing to the SERS signals)46. In our case, since the estimated close contact concentration is in good agreement with the experimental saturation region (i.e., all the R6G molecules present are contributing to SERRS) the analytical enhancement and true


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enhancement factors should be the same, and the enhancement factor values we obtained are consistent with that hypothesis.

3.3. SERRS of R6G aerosols For our aerosol studies, a sample is prepared in the same way as for the bulk study, by mixing 5 x 10-7 M R6G dye in water with 2 x 10-11 M 40 nm Ag NPs suspended in water and 3 x 10-2 M NaCl solution. Charged droplets of this mixture are produced and introduced into the LEQ for study. Under ambient conditions the droplets dry within several milliseconds, leaving particles composed primarily of NaCl, but with small amounts of R6G, MNPs, and possibly a small amount of residual water. Our electrodynamic trap is capable of trapping particles greater than 0.5 micron in diameter under ambient airflow conditions. Therefore, for studying aerosols composed of dilute analyte concentrations, an initial concentration of an inert material, such as NaCl, was chosen to provide micron-sized particles after evaporation of the water as a convenience for trapping and interrogation. SEM images of aerosols generated from droplets of analyte and MNPs were obtained in a related, but separate study, which have been presented elsewhere47. Examples of measured SERRS spectra from 5, 9 and 11 µm diameter particles are plotted in Fig. 5 along with the SERRS spectrum of 10-8 M bulk, liquid sample from Fig. 3 for comparison. The spectra for aerosol and bulk samples are all recorded using 100 mW of laser power (2kW/cm2 fluence) with 10 s exposure time.

Comparison of the spectra of the bulk

sample and the aerosol particles show good agreement in terms of the Raman peak positions, and also the fluorescence shape, for the 11 µm particle whose fluorescence is not completely quenched.



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Figure 5. SERRS spectra of aerosol particles generated from suspensions of 5 x 10-7 M R6G with 2 x 10-11 M 40 nm Ag NPs and 3 x 10-2 M of NaCl. The SERRS spectrum of R6G bulk sample is also plotted from Fig. 3 for comparison. All spectra were measured at 2 kW/cm2 laser fluence and 10 s exposure time. The positions of the Raman peaks are in good agreement between the SERRS spectra of the bulk (liquid) sample and all of the aerosol particles.

Droplets are generated from solutions of R6G, ranging in concentration from 5 x 10-9 M to 5 x 10-6 M in combination with 1.5 x 10-11 M 40 nm Ag NPs and 3 x10-2 M NaCl solution. The initial diameter of the generated droplets ranges from 35 µm (22 pL) to 50 µm (65pL). These droplets quickly dry to resultant particles with diameters of 3 to 5 µm, which are estimated by imaging. About 20 particles are generated at each concentration and for particles that exhibited SERRS, the mean of the area under the curve, after fluorescence subtraction, is computed for three of the Raman peaks at 617, 1514 and 1657 cm-1, and these values are plotted in Fig. 6 with error bars of one standard deviation.

The spread in the data is wide, largely due to the


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distribution in particle sizes (mass varies by a factor ~ 5 for particles ranging in size between 3 to 5 µm in diameter) as well as possible variation in particle morphology in terms of proximities of R6G molecules, to MNPs among NaCl molecules. However, the SERRS signal shows a trend consistent with a linear response (slope of unity) at lower concentrations, but clearly exhibits saturation and quenching at concentrations higher than 10-7 M.

For 40 nm Ag NPs,

approximately 5.3 x 103 R6G molecules

Figure 6. SERRS signal measured at 617, 1514 and 1657 cm-1 bands for aerosol particles ranging in size from 3 to 5 µm in diameter. R6G concentration was varied from 5 x 10-9 M to 5 x 10-6 M and mixed with 2 x 10-12 M 40 nm Ag and 3 x 10-2 M of NaCl. The number of R6G molecules present in each particle is plotted as a secondary x-axis on top. Particles are all composed of 99.6% NaCl and 300 Ag NPs with a varying number of R6G molecules ranging from 105 to 108.



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can be estimated in close packing contact (single layer) with each MNP which would be equivalent to an upper bound R6G concentration of 7 x 108 M for a 2 x10-11 M Ag NPs solution. This behavior is similar to the observed behavior for the bulk R6G measurement (Fig. 4), and this estimated R6G saturation concertation is in good agreement with the observed experimental saturation region. As the concentration of the analyte is further increased beyond the saturation region, quenching can occur due to re-absorption of the elastic and Raman scattering light. The corresponding fluorescence of the particles was also analyzed, but particle-to-particle fluorescence intensities exhibited such wide variation that no conclusion can be drawn for the fluorescence dependence on analyte concentration. As discussed earlier, the intensity of the fluorescence is dependent on the spacing of the analyte molecule and MNP, which is indirectly controlled by their concentrations and the adsorption strength, relationships which are too complex to decouple in our current study.

However, both quenching and enhancement in

fluorescence was observed as a function of MNP concentration in an earlier study exploring fluorescence response from aerosolized composite particles of MNPs and fluorophores47. Considering the composition of the particles discussed in Fig. 6, even for particles generated from the highest concentration of R6G, the mass fraction composition of a nominally 4 µm aerosol particle is 99.68 % NaCl, 0.18% Ag NPs and 0.14% R6G. Therefore, NaCl played both the part of the electrolyte in the experiment, as well as the inert material matrix needed to generate aerosols large enough to be easily trapped and observed by the LEQ in atmospheric conditions. The estimated number of MNPs in each aerosol particle is around 300 and the number of R6G molecules ranges from 105 (equivalent to 80 ag in mass, or a 50 nm diameter sphere) up to 108 (equivalent to 80 fg in mass, or a 500 nm diameter sphere) per aerosol particle as shown by the top x-axis label of Fig. 6. The ratio of R6G molecules to each MNP is 330 at


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our limit of detection concentration of 5 x 10-9 M. In a similar manner to the determinations made for the bulk liquid data, a total SERRS cross-section can be calculated for aerosols by comparing the measured SERRS intensity of R6G particles to spontaneous Raman intensity of droplets of neat ethyl cinnamate with its known cross-section and taking into account the respective numbers of molecules. Droplets of ethyl cinnamate are generated between 25 and 50 µm in diameter and their sizes are measured by direct imaging. The measured spontaneous Raman intensity at 1638 cm-1 normalized to the number of molecules present in the ethyl cinnamate droplet is compared to the SERRS intensity at 1504 cm-1 for R6G normalized to the number of molecules present in the particle, and scaled to the known cross-section of ethyl cinnamate. This yield a SERRS cross-section of 8.4 x 10-21 cm2 for R6G particles. Similar to the bulk study, by normalizing our experimental SERRS cross-section to the R6G resonance Raman cross-section reported by Shim et al.37 at 532 nm of 2.2 x 10-23 cm2 for the 1504 cm-1 band, yields a SERS analytical enhancement factor of 380. Similarly, by comparing our SERRS cross-section to the spontaneous Raman cross-section that is scaled for the wavelength difference, reported by Rue et al.46 of 2.9 x 10-26 cm2 at 633 nm for the same 1504 cm-1 band, a SERS and resonance Raman combined (SERRS) analytical enhancement factor of 1.4 x 105 is obtained for aerosols of R6G. For analyte concentrations within the range of 5 x 10-6 M to 5 x 10-8 M, approximately 60% of the generated R6G particles exhibit SERRS, while the remainder exhibit only fluorescence. When the analyte concentration was lowered to 5x10-9 M, the percentage of particles exhibiting SERRS decreased to 10% and only fluorescence to 35% with the remaining 55% exhibit no observable signal. One possibility is that the high concentration of NaCl molecules in our study is inhibiting the SERS enhancement. To test this hypothesis, aerosols were generated from



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mixtures where the concentration of Ag NPs and R6G (10-7 M) was kept constant, and the concentration of NaCl was varied from 0.1 M to 0.01 M. The SERRS signal at 617 and 1657 cm-1 from these aerosol particles is plotted in Fig. 7, showing an inverse correlation with the NaCl concentration, with a slope of

-0.6 and -0.9 respectively. The experiment could not be

extended to lower concentrations of NaCl, as the resultant generated particles were too small to trap and analyze. Other studies using a hydrophobic coating to concentrate the analyte by evaporation to a small area of SERS substrate48 and evaporating analyte and MNP colloid on specially coated substrate called SLIP49 have been able to attain sensitivity of a few molecules. We plan to extend our work by replacing NaCl with an inert material such as nano-sized silica particles that should not inhibit the SERRS signal. Additionally, we plan to study particles consisting of an inert core with a surface coating of R6G and MNPs.

Figure 7. SERRS signal measured at 617 and 1657 cm-1 bands for aerosol particles ranging in size from 3 to 5 µm in diameter. R6G concentration was kept constant at 5 x 10-7 M, mixed with 2 x 10-12 M 40 nm Ag while the concentration of NaCl was varied from 0.01 M to 0.1 M. The Raman signal decreased with increasing NaCl concentration with a slope of -0.6 and -0.9 for the


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two Raman bands respectively, suggesting that the presence of NaCl at these concentrations may be inhibiting SERRS enhancement.

4. CONCLUSION We have developed a Raman measurement system to study both spontaneous Raman and SERS from bulk samples of suspensions and from single aerosol particles held in an LEQ trap. SERRS measurements on R6G as bulk sample (solution) was carried out, resulting in a total analytical enhancement factor of 7 x 105. SERRS signals have been also observed and quantified from 3 to 5 µm aerosols composed of R6G, Ag NPs and NaCl. Typically, SERS experiments are conducted by placing molecules on a specialized substrate or as colloids, as in our bulk study, but to our knowledge this is the first time SERS has been observed from individual suspended aerosol particles consisting of analyte molecules and MNPs. A SERRS analytical enhancement of 2 x 105 was observed for suspended individual particles with a detection limit of 105 R6G molecules and 300 Ag NPs. Future experiments to improve the current detection limit by substituting NaCl with other inert materials are planned. Tailoring the concentration of the MNPs or using faceted MNPs may also greatly improve the sensitivity of the SERS measurement50. We plan to study SERS from other surrogate aerosols that are representative of materials of interest for the defense and atmospheric communities and quantify their SERS enhancement. While R6G was chosen as a model molecule to perform initial studies, the crosssection of R6G may be orders of magnitude higher than more relevant materials. Such studies



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will allow us to evaluate feasibility of implementing our technique for real-time detection of atmospheric and threat agent aerosols in situ.

AUTHOR INFORMATION Corresponding Author *[email protected]

ACKNOWLEDGEMENT This work was funded by the Office of Naval Research ONR 61153N. The authors would like to thank Gary Kushto (Naval Research Laboratory) for discussions and help with sample preparation. Would like to extend our thanks to John Tucker and Paul Lane of Naval Research Laboratory for reviewing the manuscript and providing helpful feedback.

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Surface Enhanced Raman Spectroscopy of Individual Suspended

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