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Ultralow concentration β-carotene molecule detection by liquid-core optical fiber resonance Raman spectroscopy
Vibrational Spectroscopy 62 (2012) 7–9
Contents lists available at SciVerse ScienceDirect
Vibrational Spectroscopy journal homepage: www.elsevier.com/locate/vibspec
Short communication
Ultralow concentration -carotene molecule detection by liquid-core optical fiber resonance Raman spectroscopy Jian-Hua Yin a,b,∗ , Zhi-Yan Xiao d , Zuo-Wei Li c a
Department of Biomedical Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China Department of Physics, Oakland University, Rochester, MI 48309, USA College of Physics, Jilin University, Changchun 130012, China d Department of Radiation Oncology, Washington University School of Medicine, St. Louis, MO 63110, USA b c
a r t i c l e
i n f o
Article history: Received 13 September 2011 Received in revised form 24 May 2012 Accepted 29 May 2012 Available online 3 June 2012 Keywords: Liquid-core optical fiber Resonance Raman scattering Ultralow concentration -Carotene
a b s t r a c t Liquid-core optical fiber-based resonance Raman scattering (LCOF-RRS) that can enhance Raman intensity 109 times has been used to measure -carotene in CS2 with the concentration ranging from 10−7 to 10−16 M. The 514.5 nm excited resonance Raman spectra (1 band) of -carotene superimposed on a fluorescence background are clearly distinct. The corresponding Raman scattering cross sections (RSCS) were evaluated by comparing the intensity of LCOF-RRS with the intensity of solution fluorescence of -carotene. Due to the resonance Raman effect and dilution effect, the RSCS enhances remarkably and is comparable with the fluorescence cross section of -carotene. This work shows that ultralow concentration molecules can be detected by the LCOF-RRS technique. The extreme sensitivity attainable and related structure information of molecule detected by the LCOF-RRS make it a potential analytical tool at ultralow concentration. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Detecting interesting molecules at ultralow concentration with high sensitivity and molecular specificity is of great scientific and practical significance in many fields such as life science, medicine, biology, pharmacology, chemistry, and environmental science [1]. Current methods for probing ultralow concentration and single molecules are restricted. Currently, the most widely used methods are fluorescence and Raman spectroscopy [2]. Fluorescence spectroscopy provides ultra-high sensitivity and a very good signalto-noise ratio [3], however, the amount of molecular information which can be obtained from the broad fluorescence bands is limited, particularly at room temperature [2]. In most cases, to detect and to identify single molecules, the molecules must be labeled by fluorescent dye molecules to achieve high fluorescence quantum yields enough to distinguish spectral properties. Raman spectroscopy is advantageous because it can provide rich information on molecular structure, and avoid photobleaching. Because of its high information content on chemical structure, Raman scattering is a very promising technique for detecting ultralow concentration molecules. For example, surface-enhanced Raman scattering (SERS) (a related method of Raman spectroscopy) is a highly effective analytical tool for ultralow concentration molecule detection
∗ Corresponding author. E-mail address: dr [email protected] (J.-H. Yin). 0924-2031/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.vibspec.2012.05.014
and single molecule detection by means of enhancing Raman intensities by 103 –105 times, even as high as 1014 times [4,5]. Similar with SERS technique, the liquid-core optical fiber (LCOF) technique can enhance Raman intensity by 103 times [6–9]. If resonance Raman scattering is excited in LCOF (LCOF-RRS), the scattering intensity of the special Raman band can be raised by 9 orders of magnitude [10,11]. Based on these results, LCOF-RRS of -carotence in CS2 was obtained at low concentrations by 10−13 M [10]. However, no Raman scattering cross sections (RSCS) was involved in this work [8–10]. When RSCS of -carotene was investigated by using LCOF [12,13], however, the detecting concentration was limited at 10−10 M [12] and 10−11 M [13], respectively. In the present paper, LCOF-RRS was used to detect the RSCS of -carotene at ultralow concentrations as low as 10−16 M. Theoretical explanations for the enhancement of RSCS are proposed. 2. Experimental Before making the LCOF, sufficient cleaning was performed carefully for all the containers used in the experiment, including pipettes, the quartz glass tube that was used for manufacturing the hollow core optical fiber, and the manufactured hollow core optical fiber. As a classic and efficient RRS molecule, -carotene with more than 97% purity was purchased from Sigma without further purification. It was dissolved in purified carbon disulfide (CS2 ) solvent. The initial solution was successively diluted to new solutions
8
J.-H. Yin et al. / Vibrational Spectroscopy 62 (2012) 7–9
Fig. 1. Schematic diagram of LCOF.
with various concentrations ranging from 10−7 to 10−16 M, which were filled into the hollow quartz fibers to make up the LCOFs with an inner diameter of 200 m and length of 1.6 m. Both ends of LCOF were sealed with a special “seal cell” consisting of windows which can prevent the liquid from flowing and avert LCOF from damaging. The structure drawing of LCOF was shown in Fig. 1. Raman spectra were measured using front-face collection geometry. In this technique, exciting light enters into one end of LCOF and scattered light is collected from the other end. This method is useful for getting large signal from opaque, turbid, or highly absorbing materials. CS2 was adopted as solvent mainly because the refractive index of CS2 is higher than that of quartz [14], which complies with the principles of light conduction in optical fibers. Raman spectra were collected by DILOR OMARS 89 model Raman spectrometer at room temperature with resolution of 2 cm−1 . The 514.5 nm line of an Ar+ laser with 50 mW power was applied to excite Raman scattering. The integration time was 60 s for one measurement. Visible absorption spectrum was collected out using VaRian Cary 50 spectrophotometer at room temperature. 3. Results Fig. 2 shows the visible absorption spectrum of -carotene in CS2 with concentration of 10−4 M. The exciting wavelength of 514.5 nm exactly locates in the broad absorption band that is mainly due to the → * electric transition of C C from the ground electronic state (S0 ) to the lowest allowed excited state (S2 ) [15–17]. Thus, the S2 ← S0 transition can be resonantly excited by the 514.5 nm laser and rigorous RRS (the laser wavelength is near the absorption maximum) is expected to be observed. The probability of energy exchange between an incoming photon and vibrational levels of the molecule would be dramatically increased and the intensities of related Raman lines can be enhanced by a factor of 105 –106 when compared to a completely spontaneous system [10,11,18].
Fig. 2. Visible absorption spectrum of -carotene in CS2 at concentration of 10−4 M. The 514.5 nm laser falls within this electronic absorption band of the transition S2 ← S0 .
Fig. 3 presents the 514.5 nm excited resonance Raman spectra of -carotene in CS2 at various concentrations from 10−7 to 10−16 M, which shows the remarkable enhancement in Raman intensity of 1 (1514 cm−1 ) band by using LCOF-RRS technique. The stronger signal component of the spectra in Fig. 3A is a spectrally broad background originating from fluorescence of -carotene. This is because the 1 band falls within the range of the maximum of the solution fluorescence of -carotene in CS2 , around 558 nm [15–17,19]. The fluorescence emission envelope is clearly visible. The characteristic Raman band (1 ) of the -carotene [18] superimposing upon the fluorescence background appears as very remarkable bump on the trace, meaning that LCOF-RRS intensity of 1 band achieves the level of high fluorescence intensity for -carotene. Therefore, this result suggests that the cross section of the LCOF-RRS of 1 band approaches that of fluorescence based on the method [20] of comparing LCOF-RRS intensity with fluorescence background in the range of 10−7 -10−16 M. After the fluorescence background has been fitted with a sixth-order polynomial and subtracted from the original spectra [20,21], the 1 Raman band of -carotene can be resolved with a high signal-to-noise ratio, as plotted in Fig. 3B.
4. Discussion The size of the enhancement factor of the effective LCOFRRS cross section (intensity) is the key to explain the results above. Raman scattering is a very weak effect and RSCS is between 10−30 and 10−26 cm2 molecule−1 under normal conditions [5,11,22]. For -carotene, the RSCS of 1 band is approximately 10−27 cm2 molecule−1 [11,23] under normal conditions due to the high content of C C double bonds, which is much less than its effective fluorescence cross section, 10−17 t o 10−16 cm2 molecule−1 [5,17]. In previously published work, Tian [12] and Ouyang [13] et al. investigated differential RSCSs of -carotene in H2 O and binary solvents of hexane and CS2 , respectively, under preresonance Raman scattering condition with back-face collection geometry. In the latter’s report, although CS2 solvent was adopted, mixing it with hexane resulted in a blue shift in the absorption band [19,24]. Therefore, in both cases [12,13], the exciting wavelength of 514.5 nm came within the low frequency wings (pre-resonance Raman scattering) but not under the electronic absorption band of S2 ← S0 (rigorous RRS). Because of the limited pre-resonance effect, the RSCS enhancement of 1 band [12,13] could be due to the dilution effect. With the decrease of solution concentration, Raman intensity that was normalized to respective concentration increased [13,20,22], which is proportional to the RSCS [20]. Therefore LCOF-RRS cross section of 1 band of -carotene increased with diluting solution. In essence, the degree of delocalization of -electron in -carotene chain enhances with decreasing concentration and subsequently provides a high electron–photon coupling. These changes led to not only the increases in RSCS [13,25] but also the red shift of 1 band [8,9]. As a result, the differential RSCS of 1 band increased up to 1.22 × 10−21 [12] and 2.63 × 10−20 cm2 molecule−1 sr−1 [13] at 10−10 M and 10−11 M, respectively. However, an enhancement factor of 103 –104 is still necessary in order to achieve the fluorescence cross section level. In this case, the resonance Raman effect must be considered into the explanation. It’s well known that the RRS can achieve adequate conversion rates from excitation laser photons to Raman photons and enhance RSCS by 105 –106 folds, which makes sense to get detectable resonance Raman signal on the fluorescence background, as a result in Fig. 3. Hence it is concluded that the enhancement in RSCS of 1 band at ultralow concentrations is mainly due to the conjunct action of the resonance Raman effect and the dilution effect. It has
J.-H. Yin et al. / Vibrational Spectroscopy 62 (2012) 7–9
9
Fig. 3. LCOF-resonance Raman spectra of -carotene in CS2 at concentrations of (a) 10−7 M, (b) 10−8 M, (c) 10−9 M, (d) 10−10 M, (e) 10−11 M, (f) 10−12 M, (g) 10−13 M, (h) 10−14 M, (i) 10−15 M and (j) 10−16 M before (A) and after (B) subtracting fluorescence background from raw data. The dotted line at 1514 cm−1 marks the characteristic Raman peak of the -carotene, 1 . The other wavenumber region without clear Raman bands of -carotene is not shown here. In (A), the spectral intensity at relative higher concentration looks weaker than that at lower concentration, but the fluorescence background is actually stronger than that at lower concentration. The stronger fluorescence bump at relative higher concentration almost disappears the Raman signal and makes the curve flat, which results in the Raman and fluorescence intensities look weaker than those at lower concentrations.
to be pointed out that the dilution effect may lose efficacy at the concentrations being lower than 10−8 to 10−11 M, which is ascribed that the dilution effect is based on the Onsager model [12,13,22,26] which hypothesizes that each solute molecule is surrounded by a number of solvent molecules. According to the Gibbs equation or the Langmuir isotherm, the amount of solvent molecules involving one solute molecule might become saturated in the Onsager system when the concentration is lower than 10−8 to 10−11 M [27]. The saturated concentration would be various for different solute molecule [27]. Based on both of the resonance Raman effect and the dilution effect, it’s understandable that the fluorescence is gradually replaced by LCOF-RRS with decreasing solution concentration and the LCOF-RRS signal becomes more and more distinct, as shown in Fig. 3. The LCOF-RRS results are different from those obtained under both pre-resonance conditions [12,13], in which it is difficult to get a Raman signal below 10−10 M and 10−11 M, respectively. This work demonstrates that low solution concentrations are able to not only minimize the -carotene fluorescence under RRS condition, but also optimize relative intense of LCOF-RRS spectra.
References [1] [2] [3] [4] [5]
[6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]
5. Conclusions LCOF-RRS, as a novel detecting technique, was applied to ultralow concentration molecule detection. The 514.5 nm excited resonance Raman spectra of -carotene in CS2 with the concentration in the range from 10−7 to 10−16 M have been obtained by LCOF-RRS technique. LCOF-RRS cross section of -carotene approaches the level of its effective fluorescence cross section. The significant enhancement of RSCS is mainly due to the resonance Raman effect and dilution effect. LCOF-RRS combines the efficiency of high sensitive laser fluorescence spectroscopy and related structural information of vibrational spectroscopy to make the simple and inexpensive experimental technique. LCOF-RRS is a promising new powerful analytical tool and has numerous potential applications for quantitative and dynamic studies of ultralow concentration molecule detection.
[18] [19] [20] [21] [22] [23]
[24] [25] [26] [27]
M.D. Barnes, W.B. Whitten, J.M. Ramsey, Anal. Chem. 67 (1995) 418A. S. Nie, S.R. Emory, Science 275 (1997) 1102. M. Orrit, J. Chem. Phys. 117 (2002) 10938. K. Kneipp, H. Kneipp, I. Itzkan, R.R. Dasari, M.S. Feld, Chem. Rev. 99 (1999) 2957. K. Kneipp, H. Kneipp, I. Itzkan, R.R. Dasari, M.S. Feld, in: C. Zander, J. Enderlein, R.A. Keller (Eds.), Single-Molecule Detection in Solution: Methods and Applications, Wiley-VCH, 2002. G.E. Walrafen, J. Stone, Appl. Spectrosc. 26 (1972) 585. W. Wang, Q.S. He, T. Wang, M.Z. Fen, Y.M. Liao, G.X. Ran, Anal. Chem. 64 (1992) 22. J.H. Yin, S.Q. Gao, X.F. Xu, Z.W. Li, Chem. J. Chin. Univ. 23 (2002) 2300. G. Shuqin, H. Baokun, L. Zuowei, Chem. Phys. Lett. 392 (2004) 123. Z.W. Li, J.N. Li, S.Q. Gao, Jpn. J. Appl. Phys. 37 (1998) 1889. R.L. McCreery, Raman Spectroscopy for Chemical Analysis, John Wiley & Sons, Inc., 2000. Y.J. Tian, J. Zuo, L.Y. Zhang, Z.W. Li, S.Q. Gao, G.H. Lu, Appl. Phys. B 87 (2007) 727. S.L. Ouyang, C.L. Sun, M. Zhou, Z.L. Li, Z.W. Men, D.F. Li, Z.W. Li, S.Q. Gao, G.H. Lu, J. Raman Spectrosc. 41 (2010) 1650. J.H. Yin, Z.W. Li, Spectrochim. Acta A 61 (2005) 495. T. Gillbro, R.J. Cogdell, Chem. Phys. Lett. 158 (1989) 312. M. Mimuro, U. Nagashima, S. Takaichi, Y. Nishimura, I. Yamazaki, T. Katoh, Biochim. Biophys. Acta 1098 (1992) 271. A.P. Shreve, J.K. Trautman, T.G. Owens, A.C. Albrecht, Chem. Phys. Lett. 178 (1991) 89. S. Saito, M. Tasumi, C.H. Eugster, J. Raman Spectrosc. 14 (1983) 299. T. Itoh, Chem. Phys. Lett. 505 (2011) 96. B. Pettinger, K. Krischer, J. Electron Spectrosc. Relat. Phenom. 45 (1987) 133. I.V. Ermakov, M.R. Ermakova, R.W. McClane, W. Gellermann, Opt. Lett. 26 (2001) 1179. J.H. Yin, S.Q. Gao, Z.W. Li, Y.N. Yu, G.H. Lu, Y.J. Tian, J. Raman Spectrosc. 35 (2004) 1042. Differential Raman cross section, ˇ (cm2 molecule−1 sr−1 ) = d/d˝, is integrated over the bandwidth, ˝ represents the solid angle of collection. is the integrated RSCS and include scattering in all directions with units of square centimeters per molecule. So the RSCS of -carotene, = ˇ d˝. Because ˇ is about 10−23 cm2 molecule−1 sr−1 under 514.5 nm excited resonance condition with a 105 enhancement in RSCS [11], under normal condition is evaluated at 10−27 cm2 molecule−1 after integrating ˇ. Y. Hu, H. Hashimoto, G. Moine, U. Hengartner, Y. Koyama, J. Chem. Soc., Perkin Trans. 2 (1997) 2699. T.W. Hagler, K. Pakbaz, K.F. Voss, A.J. Heeger, Phys. Rev. B 44 (1991) 8652. L. Onsager, J. Am. Chem. Soc. 58 (1936) 1486. J. Yin, H. Watarai, Anal. Bioanal. Chem. 389 (2007) 895.
Contents lists available at SciVerse ScienceDirect
Vibrational Spectroscopy journal homepage: www.elsevier.com/locate/vibspec
Short communication
Ultralow concentration -carotene molecule detection by liquid-core optical fiber resonance Raman spectroscopy Jian-Hua Yin a,b,∗ , Zhi-Yan Xiao d , Zuo-Wei Li c a
Department of Biomedical Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China Department of Physics, Oakland University, Rochester, MI 48309, USA College of Physics, Jilin University, Changchun 130012, China d Department of Radiation Oncology, Washington University School of Medicine, St. Louis, MO 63110, USA b c
a r t i c l e
i n f o
Article history: Received 13 September 2011 Received in revised form 24 May 2012 Accepted 29 May 2012 Available online 3 June 2012 Keywords: Liquid-core optical fiber Resonance Raman scattering Ultralow concentration -Carotene
a b s t r a c t Liquid-core optical fiber-based resonance Raman scattering (LCOF-RRS) that can enhance Raman intensity 109 times has been used to measure -carotene in CS2 with the concentration ranging from 10−7 to 10−16 M. The 514.5 nm excited resonance Raman spectra (1 band) of -carotene superimposed on a fluorescence background are clearly distinct. The corresponding Raman scattering cross sections (RSCS) were evaluated by comparing the intensity of LCOF-RRS with the intensity of solution fluorescence of -carotene. Due to the resonance Raman effect and dilution effect, the RSCS enhances remarkably and is comparable with the fluorescence cross section of -carotene. This work shows that ultralow concentration molecules can be detected by the LCOF-RRS technique. The extreme sensitivity attainable and related structure information of molecule detected by the LCOF-RRS make it a potential analytical tool at ultralow concentration. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Detecting interesting molecules at ultralow concentration with high sensitivity and molecular specificity is of great scientific and practical significance in many fields such as life science, medicine, biology, pharmacology, chemistry, and environmental science [1]. Current methods for probing ultralow concentration and single molecules are restricted. Currently, the most widely used methods are fluorescence and Raman spectroscopy [2]. Fluorescence spectroscopy provides ultra-high sensitivity and a very good signalto-noise ratio [3], however, the amount of molecular information which can be obtained from the broad fluorescence bands is limited, particularly at room temperature [2]. In most cases, to detect and to identify single molecules, the molecules must be labeled by fluorescent dye molecules to achieve high fluorescence quantum yields enough to distinguish spectral properties. Raman spectroscopy is advantageous because it can provide rich information on molecular structure, and avoid photobleaching. Because of its high information content on chemical structure, Raman scattering is a very promising technique for detecting ultralow concentration molecules. For example, surface-enhanced Raman scattering (SERS) (a related method of Raman spectroscopy) is a highly effective analytical tool for ultralow concentration molecule detection
∗ Corresponding author. E-mail address: dr [email protected] (J.-H. Yin). 0924-2031/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.vibspec.2012.05.014
and single molecule detection by means of enhancing Raman intensities by 103 –105 times, even as high as 1014 times [4,5]. Similar with SERS technique, the liquid-core optical fiber (LCOF) technique can enhance Raman intensity by 103 times [6–9]. If resonance Raman scattering is excited in LCOF (LCOF-RRS), the scattering intensity of the special Raman band can be raised by 9 orders of magnitude [10,11]. Based on these results, LCOF-RRS of -carotence in CS2 was obtained at low concentrations by 10−13 M [10]. However, no Raman scattering cross sections (RSCS) was involved in this work [8–10]. When RSCS of -carotene was investigated by using LCOF [12,13], however, the detecting concentration was limited at 10−10 M [12] and 10−11 M [13], respectively. In the present paper, LCOF-RRS was used to detect the RSCS of -carotene at ultralow concentrations as low as 10−16 M. Theoretical explanations for the enhancement of RSCS are proposed. 2. Experimental Before making the LCOF, sufficient cleaning was performed carefully for all the containers used in the experiment, including pipettes, the quartz glass tube that was used for manufacturing the hollow core optical fiber, and the manufactured hollow core optical fiber. As a classic and efficient RRS molecule, -carotene with more than 97% purity was purchased from Sigma without further purification. It was dissolved in purified carbon disulfide (CS2 ) solvent. The initial solution was successively diluted to new solutions
8
J.-H. Yin et al. / Vibrational Spectroscopy 62 (2012) 7–9
Fig. 1. Schematic diagram of LCOF.
with various concentrations ranging from 10−7 to 10−16 M, which were filled into the hollow quartz fibers to make up the LCOFs with an inner diameter of 200 m and length of 1.6 m. Both ends of LCOF were sealed with a special “seal cell” consisting of windows which can prevent the liquid from flowing and avert LCOF from damaging. The structure drawing of LCOF was shown in Fig. 1. Raman spectra were measured using front-face collection geometry. In this technique, exciting light enters into one end of LCOF and scattered light is collected from the other end. This method is useful for getting large signal from opaque, turbid, or highly absorbing materials. CS2 was adopted as solvent mainly because the refractive index of CS2 is higher than that of quartz [14], which complies with the principles of light conduction in optical fibers. Raman spectra were collected by DILOR OMARS 89 model Raman spectrometer at room temperature with resolution of 2 cm−1 . The 514.5 nm line of an Ar+ laser with 50 mW power was applied to excite Raman scattering. The integration time was 60 s for one measurement. Visible absorption spectrum was collected out using VaRian Cary 50 spectrophotometer at room temperature. 3. Results Fig. 2 shows the visible absorption spectrum of -carotene in CS2 with concentration of 10−4 M. The exciting wavelength of 514.5 nm exactly locates in the broad absorption band that is mainly due to the → * electric transition of C C from the ground electronic state (S0 ) to the lowest allowed excited state (S2 ) [15–17]. Thus, the S2 ← S0 transition can be resonantly excited by the 514.5 nm laser and rigorous RRS (the laser wavelength is near the absorption maximum) is expected to be observed. The probability of energy exchange between an incoming photon and vibrational levels of the molecule would be dramatically increased and the intensities of related Raman lines can be enhanced by a factor of 105 –106 when compared to a completely spontaneous system [10,11,18].
Fig. 2. Visible absorption spectrum of -carotene in CS2 at concentration of 10−4 M. The 514.5 nm laser falls within this electronic absorption band of the transition S2 ← S0 .
Fig. 3 presents the 514.5 nm excited resonance Raman spectra of -carotene in CS2 at various concentrations from 10−7 to 10−16 M, which shows the remarkable enhancement in Raman intensity of 1 (1514 cm−1 ) band by using LCOF-RRS technique. The stronger signal component of the spectra in Fig. 3A is a spectrally broad background originating from fluorescence of -carotene. This is because the 1 band falls within the range of the maximum of the solution fluorescence of -carotene in CS2 , around 558 nm [15–17,19]. The fluorescence emission envelope is clearly visible. The characteristic Raman band (1 ) of the -carotene [18] superimposing upon the fluorescence background appears as very remarkable bump on the trace, meaning that LCOF-RRS intensity of 1 band achieves the level of high fluorescence intensity for -carotene. Therefore, this result suggests that the cross section of the LCOF-RRS of 1 band approaches that of fluorescence based on the method [20] of comparing LCOF-RRS intensity with fluorescence background in the range of 10−7 -10−16 M. After the fluorescence background has been fitted with a sixth-order polynomial and subtracted from the original spectra [20,21], the 1 Raman band of -carotene can be resolved with a high signal-to-noise ratio, as plotted in Fig. 3B.
4. Discussion The size of the enhancement factor of the effective LCOFRRS cross section (intensity) is the key to explain the results above. Raman scattering is a very weak effect and RSCS is between 10−30 and 10−26 cm2 molecule−1 under normal conditions [5,11,22]. For -carotene, the RSCS of 1 band is approximately 10−27 cm2 molecule−1 [11,23] under normal conditions due to the high content of C C double bonds, which is much less than its effective fluorescence cross section, 10−17 t o 10−16 cm2 molecule−1 [5,17]. In previously published work, Tian [12] and Ouyang [13] et al. investigated differential RSCSs of -carotene in H2 O and binary solvents of hexane and CS2 , respectively, under preresonance Raman scattering condition with back-face collection geometry. In the latter’s report, although CS2 solvent was adopted, mixing it with hexane resulted in a blue shift in the absorption band [19,24]. Therefore, in both cases [12,13], the exciting wavelength of 514.5 nm came within the low frequency wings (pre-resonance Raman scattering) but not under the electronic absorption band of S2 ← S0 (rigorous RRS). Because of the limited pre-resonance effect, the RSCS enhancement of 1 band [12,13] could be due to the dilution effect. With the decrease of solution concentration, Raman intensity that was normalized to respective concentration increased [13,20,22], which is proportional to the RSCS [20]. Therefore LCOF-RRS cross section of 1 band of -carotene increased with diluting solution. In essence, the degree of delocalization of -electron in -carotene chain enhances with decreasing concentration and subsequently provides a high electron–photon coupling. These changes led to not only the increases in RSCS [13,25] but also the red shift of 1 band [8,9]. As a result, the differential RSCS of 1 band increased up to 1.22 × 10−21 [12] and 2.63 × 10−20 cm2 molecule−1 sr−1 [13] at 10−10 M and 10−11 M, respectively. However, an enhancement factor of 103 –104 is still necessary in order to achieve the fluorescence cross section level. In this case, the resonance Raman effect must be considered into the explanation. It’s well known that the RRS can achieve adequate conversion rates from excitation laser photons to Raman photons and enhance RSCS by 105 –106 folds, which makes sense to get detectable resonance Raman signal on the fluorescence background, as a result in Fig. 3. Hence it is concluded that the enhancement in RSCS of 1 band at ultralow concentrations is mainly due to the conjunct action of the resonance Raman effect and the dilution effect. It has
J.-H. Yin et al. / Vibrational Spectroscopy 62 (2012) 7–9
9
Fig. 3. LCOF-resonance Raman spectra of -carotene in CS2 at concentrations of (a) 10−7 M, (b) 10−8 M, (c) 10−9 M, (d) 10−10 M, (e) 10−11 M, (f) 10−12 M, (g) 10−13 M, (h) 10−14 M, (i) 10−15 M and (j) 10−16 M before (A) and after (B) subtracting fluorescence background from raw data. The dotted line at 1514 cm−1 marks the characteristic Raman peak of the -carotene, 1 . The other wavenumber region without clear Raman bands of -carotene is not shown here. In (A), the spectral intensity at relative higher concentration looks weaker than that at lower concentration, but the fluorescence background is actually stronger than that at lower concentration. The stronger fluorescence bump at relative higher concentration almost disappears the Raman signal and makes the curve flat, which results in the Raman and fluorescence intensities look weaker than those at lower concentrations.
to be pointed out that the dilution effect may lose efficacy at the concentrations being lower than 10−8 to 10−11 M, which is ascribed that the dilution effect is based on the Onsager model [12,13,22,26] which hypothesizes that each solute molecule is surrounded by a number of solvent molecules. According to the Gibbs equation or the Langmuir isotherm, the amount of solvent molecules involving one solute molecule might become saturated in the Onsager system when the concentration is lower than 10−8 to 10−11 M [27]. The saturated concentration would be various for different solute molecule [27]. Based on both of the resonance Raman effect and the dilution effect, it’s understandable that the fluorescence is gradually replaced by LCOF-RRS with decreasing solution concentration and the LCOF-RRS signal becomes more and more distinct, as shown in Fig. 3. The LCOF-RRS results are different from those obtained under both pre-resonance conditions [12,13], in which it is difficult to get a Raman signal below 10−10 M and 10−11 M, respectively. This work demonstrates that low solution concentrations are able to not only minimize the -carotene fluorescence under RRS condition, but also optimize relative intense of LCOF-RRS spectra.
References [1] [2] [3] [4] [5]
[6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]
5. Conclusions LCOF-RRS, as a novel detecting technique, was applied to ultralow concentration molecule detection. The 514.5 nm excited resonance Raman spectra of -carotene in CS2 with the concentration in the range from 10−7 to 10−16 M have been obtained by LCOF-RRS technique. LCOF-RRS cross section of -carotene approaches the level of its effective fluorescence cross section. The significant enhancement of RSCS is mainly due to the resonance Raman effect and dilution effect. LCOF-RRS combines the efficiency of high sensitive laser fluorescence spectroscopy and related structural information of vibrational spectroscopy to make the simple and inexpensive experimental technique. LCOF-RRS is a promising new powerful analytical tool and has numerous potential applications for quantitative and dynamic studies of ultralow concentration molecule detection.
[18] [19] [20] [21] [22] [23]
[24] [25] [26] [27]
M.D. Barnes, W.B. Whitten, J.M. Ramsey, Anal. Chem. 67 (1995) 418A. S. Nie, S.R. Emory, Science 275 (1997) 1102. M. Orrit, J. Chem. Phys. 117 (2002) 10938. K. Kneipp, H. Kneipp, I. Itzkan, R.R. Dasari, M.S. Feld, Chem. Rev. 99 (1999) 2957. K. Kneipp, H. Kneipp, I. Itzkan, R.R. Dasari, M.S. Feld, in: C. Zander, J. Enderlein, R.A. Keller (Eds.), Single-Molecule Detection in Solution: Methods and Applications, Wiley-VCH, 2002. G.E. Walrafen, J. Stone, Appl. Spectrosc. 26 (1972) 585. W. Wang, Q.S. He, T. Wang, M.Z. Fen, Y.M. Liao, G.X. Ran, Anal. Chem. 64 (1992) 22. J.H. Yin, S.Q. Gao, X.F. Xu, Z.W. Li, Chem. J. Chin. Univ. 23 (2002) 2300. G. Shuqin, H. Baokun, L. Zuowei, Chem. Phys. Lett. 392 (2004) 123. Z.W. Li, J.N. Li, S.Q. Gao, Jpn. J. Appl. Phys. 37 (1998) 1889. R.L. McCreery, Raman Spectroscopy for Chemical Analysis, John Wiley & Sons, Inc., 2000. Y.J. Tian, J. Zuo, L.Y. Zhang, Z.W. Li, S.Q. Gao, G.H. Lu, Appl. Phys. B 87 (2007) 727. S.L. Ouyang, C.L. Sun, M. Zhou, Z.L. Li, Z.W. Men, D.F. Li, Z.W. Li, S.Q. Gao, G.H. Lu, J. Raman Spectrosc. 41 (2010) 1650. J.H. Yin, Z.W. Li, Spectrochim. Acta A 61 (2005) 495. T. Gillbro, R.J. Cogdell, Chem. Phys. Lett. 158 (1989) 312. M. Mimuro, U. Nagashima, S. Takaichi, Y. Nishimura, I. Yamazaki, T. Katoh, Biochim. Biophys. Acta 1098 (1992) 271. A.P. Shreve, J.K. Trautman, T.G. Owens, A.C. Albrecht, Chem. Phys. Lett. 178 (1991) 89. S. Saito, M. Tasumi, C.H. Eugster, J. Raman Spectrosc. 14 (1983) 299. T. Itoh, Chem. Phys. Lett. 505 (2011) 96. B. Pettinger, K. Krischer, J. Electron Spectrosc. Relat. Phenom. 45 (1987) 133. I.V. Ermakov, M.R. Ermakova, R.W. McClane, W. Gellermann, Opt. Lett. 26 (2001) 1179. J.H. Yin, S.Q. Gao, Z.W. Li, Y.N. Yu, G.H. Lu, Y.J. Tian, J. Raman Spectrosc. 35 (2004) 1042. Differential Raman cross section, ˇ (cm2 molecule−1 sr−1 ) = d/d˝, is integrated over the bandwidth, ˝ represents the solid angle of collection. is the integrated RSCS and include scattering in all directions with units of square centimeters per molecule. So the RSCS of -carotene, = ˇ d˝. Because ˇ is about 10−23 cm2 molecule−1 sr−1 under 514.5 nm excited resonance condition with a 105 enhancement in RSCS [11], under normal condition is evaluated at 10−27 cm2 molecule−1 after integrating ˇ. Y. Hu, H. Hashimoto, G. Moine, U. Hengartner, Y. Koyama, J. Chem. Soc., Perkin Trans. 2 (1997) 2699. T.W. Hagler, K. Pakbaz, K.F. Voss, A.J. Heeger, Phys. Rev. B 44 (1991) 8652. L. Onsager, J. Am. Chem. Soc. 58 (1936) 1486. J. Yin, H. Watarai, Anal. Bioanal. Chem. 389 (2007) 895.