Page 1 of 11
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Analytical Chemistry
Surface Enhanced Raman Spectroscopy Enables Observations of Previously Undetectable Secondary Organic Aerosol Components at the Individual Particle Level Rebecca L. Craig†, Amy L. Bondy†, Andrew P. Ault†‡* †
Department of Chemistry, University of Michigan, Ann Arbor, Michigan, 48109, United States
Department of Environmental Health Sciences, University of Michigan, Ann Arbor, Michigan, 48109, United States Supporting Information Placeholder
‡
ABSTRACT: The first use of surface enhanced Raman spectroscopy (SERS) to detect trace organic and/or inorganic species in ambient atmospheric aerosol particles is presented. This new analytical method provides direct, spectroscopic detection of species present at attogram to femtogram levels in individual submicron atmospheric particles. An array of spectral features resulting from organic functional groups in secondary organic aerosol (SOA) material were observed in individual particles impacted on silver nanoparticle-coated substrates. The results demonstrate the complexity of organic and inorganic species in SOA formed by oxidation of biogenic volatile organic compounds (BVOCs) at the single particle level. While SOA composition is frequently assumed to be homogeneous between and within individual particles, substantial particle-to-particle variability in SOA composition and changes on scales 1 µm in diameter.12-15 However, despite this potential, Raman measurements of individual particles to date have been limited due to insufficient sensitivity to key trace species. Most Raman microspectroscopy measurements of atmospheric particles have focused on highly Raman-active modes, such as νs(SO42-), νs(NO3-), and the ν(C-H) region.14-17 These modes provide only a limited understanding of the species present and the reactions that occur within particles. To elucidate particle-phase processes requires measurements that produce greater intensity for organic modes in the fingerprint region. Additionally, the vast majority of secondary particles in the atmosphere, as well as those with the longest lifetimes, are < 1 µm in diameter. The optical and cloud nucleating properties of these small particles must be linked with chemical speciation to reduce aerosol climate uncertainties.2 Thus, methods need to measure < 1 µm particles, as well as intra-particle spatial variability with < 0.5 µm resolution.18 The use of surface enhanced Raman spectroscopy (SERS) to study atmospheric aerosols has the potential to overcome both major limitations of Raman microspectroscopy: insufficient detection limits and spatial limitations due to the diffraction limit. SERS offers strong enhancements in Raman signal (up to 1010) due to localized surface plasmon resonances (LSPRs) that activate over small spatial regions (< 10 nm).19-22 SERS offers great promise to detect key functional groups in complex mixtures. Examples of compounds present in SOA with functional groups that SERS can probe include
Figure 1. (a) Schematic of an aerosol particle impacted onto a SERS Ag nanoparticle coated quartz substrate. Optical image of aerosol particles impacted onto (a) quartz substrate and (b) Ag nanoparticle coated quartz substrate.
ACS Paragon Plus Environment
0
965 970 975 980 985 1040 1060 -1 Wavenumber (cm ) Figure 2. Non-enhanced and SERS-enhanced spectra for sulfate (left) and nitrate (right) stretching modes, derived from aerosolized (NH4)2SO4 and NaNO3. The color of the SERS-enhanced spectra corresponds to its enhancement factor, with those greater than 5 labeled above the spectra. epoxides, organosulfates, and organonitrates. These species have been observed primarily on extracted samples with liquid chromatography coupled to high resolution mass spectrometry,1,23 as well as other mass spectrometry and spectroscopy techniques.10,24
Herein, we show the first use of SERS to probe atmospheric aerosol particles. These proof-of-concept results demonstrate signal enhancement of key species, allowing for the measurement of previously undetectable functional groups and < 0.5 µm spatial variation of species within individual atmospheric particles. Figure 1 shows a schematic of an atmospheric particle deposited on quartz coated with Ag nanoparticles to generate SERS enhancement. However, not all Ag nanoparticles are SERS active,20,21 so Raman signal enhancement is not uniform across a single particle, but rather is dependent on the distance from a SERS hot spot. Because of the dependence on hot spot location, a statistically significant number of particles and particle regions must be analyzed in order to determine representative speciation within the deposited particle population. However, greater quantification will be possible as more uniform surfaces are created with more complex SERS substrate preparation that increase the number of hot spots and create more homogeneous distributions of enhanced regions. Ag nanoparticles were prepared by reduction of silver nitrate with hydroxylamine hydrochloride25 and drop coated onto quartz substrates. The Ag nanoparticles were characterized with transmission electron microscopy (TEM), confocal optical microscopy, and UVVisible spectroscopy. Details on the characterization are given in the Supporting Information. An optical microscopy (100x objective) image of atmospheric particles impacted on a quartz substrate without Ag nanoparticles is shown in Figure 1b. Most aerosol particles appear dark, except for those containing minerals that fluoresce/scatter strongly. In contrast, the Ag nanoparticles are observed as bright particles on the quartz substrate (Figure 1c), with orange features indicating particularly SERS-active regions. Atomized ammonium sulfate and sodium nitrate were used to probe enhancement due to the SERS effect at the single particle level. Figure 2 shows a comparison of non-enhanced to SERS-enhanced spectra for the ν(SO42-) and ν(NO3-) modes, with average enhancement factors of 2.0 and 3.6, respectively. Two spectra exhibited large enhancement for ν(NO3-) (enhancement factors of 13 and 30). The ν(NO3-) peak red shifted from 1067 cm-1 to 1054 cm-1, which has been observed previously in enhanced spectra of bulk aqueous nitrate and becomes more prominent with increased enhancement.26 See Supporting Information for further explanation of the peak shift, which is likely related to whether NO3- is aqueous9 and how it binds to the nanoparticle surface Ambient atmospheric particles were collected at a remote northern Michigan forested site, with sampling occurring above the forest canopy. Particles were impacted onto quartz and Ag nanoparticle coated quartz substrates using a Multi Orifice Uniform Deposit Impactor (MOUDI) during a field study at the University of Michigan
a) 1000 800 600 400 200 0
(NO3-)
100010401080
500 400 300 200 100 0
(O-H)
3400 3600
(C-H)
2000 1500 1000 500 0
b)
(C-C) 1000 800 600 400 200 0 2800 3000 140014401480 -1
Wavenumber (cm )
0 2 4 6 8 10 Enhancement Factor 20 40 27 15 18 32 17 40 72 23 17 13 17 11 12 11 14 12 9 7 5 3 1
11
(O-H)
4
(C-H)
0
8 13
(C-C)
1000
0 1 2 3 4 5 Enhancement Factor
Biological Station (UMBS) near Pellston, MI. Enhancements were observed in the Raman signal due to the SERS effect for particles analyzed with a Raman microspectrometer using a 532 nm laser source (50 mW) and averaging 2 x 1 second acquisitions. The enhancements were observed across the Raman spectrum, both for modes previously studied for atmospheric particles, (νs(NO3-), νs(C-H), and ν(O-H)),14-17 as well as modes not previously observed for organic functional groups in the fingerprint region (discussed below). Figure 3a establishes the enhancement in previously observed modes for a particle impacted on a region without Ag nanoparticles (blue) and spectra from particles deposited on the SERSactive Ag nanoparticles (red) (full spectra in Supporting Information). The enhancement in SERS spectra is shown for νs(NO3-), ν(O-H), and νs(C-H) with enhancement factors of 7, 12, and 14. A feature that is often barely detectable with conventional Raman microspectroscopy, δ(C-C), is also shown, with an enhancement factor of 8. It should be noted that the non-enhanced spectrum had the most intense signal of all spectra collected for particles not impacted on the Ag nanoparticles. Thus, comparison to that particle represents a lower limit regarding the enhancements observed. If comparisons were made to the mean non-SERS-enhanced spectrum (by intensity), the signal is more than 10 times lower and many features were not present. This indicates average enhancements of 101103 for these modes, which enable the detection of expected functional groups that have only been observed by extracting material from high volume samples.1 As evidence that enhancement was widely observed for particles on SERS-active substrates, the enhancement factor was quantified for 15 particles compared to the most intense non-SERS enhanced particle from Figure 3a. The enhancement factors for the different regions from Figure 3a are shown as a heat map in Figure 3b (gray indicates no peak for a mode in that particle). The spectra exhibited a wide range of enhancements, with ~31% of modes being enhanced by more than a factor of 10; the largest observed enhancement factor was 72. The vibrational modes highlighted by black boxes in Figure 3b, represent median enhancement factor values and correspond to the spectra shown in Figure 3a. The enhancement factor is not consistent across all vibrational modes, which can be attributed to particle-to-particle compositional variability and the non-uniform coupling of species from the different particles to the
(NO3-)
2000
12
3
Page 2 of 11
Particles
30
3000
16x10
Intensity (counts)
Enhanced (colored) Non-enhanced
-
2-
4000
NO3 Intensity (cts)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
SO4 Intensity (cts)
Analytical Chemistry
Figure 3. (a) Non-enhanced (blue) and SERS-enhanced (red) spectra of four different vibrational modes: ν(NO3-), ν(O-H), ν(C-H), and δ(C-C). The enhancement factor, 7.2, 11.7, 13.9, and 4.0, respectively, for the spectra represent median enhancement factor values for the 15 analyzed aerosol particles. (b) Calculated enhancement factor for 15 aerosol particles for the four vibrational modes of interest. Gray indicates the vibrational mode was not present in the Raman spectrum for a particular particle. The Raman spectra for vibrational modes highlighted by the black box is shown in (a).
ACS Paragon Plus Environment
1437 1512
3507 3526
2915
0.7 µm
0 400
1.1 µm
600 800 1000 1200 1400 1600 1800 2800 -1 1 second acquisition x Wavenumber (cm ) 2 accumulations
3526
2963
100
3506
998 1110 1184 1223 1337 1445
100
0
0.8 µm 1582
200
200
2915 2965
1310 1354 1509 1570 1645
1117 1182 1314 1362 1442 1506 1568 1653 1179
918 924
1051
771
2916
0.6 µm
1560
928
0 1600 1200 800 400 0
682 769
400
642
1200 800 400 0 1200 800 400 0
624 644 812 675 901 920 1046 1101 1203 1282
Analytical Chemistry
687
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Intensity (counts)
Page 3 of 11
0 400 200 0
3200
3600
Figure 4. Raman spectra of the fingerprint region (500-1800 cm-1) and higher energy region (2700-3800 cm-1) of four ambient aerosol particles exhibiting enhancement. LSPRs.27 Atmospheric particles frequently have complex internal structures,5,28 but have primarily been studied under the vacuum of electron microscopes18,28 (at times with cryo-stages) or with fluorescence microscopy on particles much larger than those present in the atmosphere (~40 um).5 The small distance where SERS enhancement is localized (
