- Email: [email protected]
Concentration of colloidal silica suspensions using fluorescence spectroscopy
Colloids and Surfaces A: Physicochem. Eng. Aspects 277 (2006) 107–110
Concentration of colloidal silica suspensions using fluorescence spectroscopy Yeomin Yoon, Richard M. Lueptow ∗ Department of Mechanical Engineering, Northwestern University, Evanston, IL, 60208, USA Received 21 October 2005; accepted 5 November 2005 Available online 15 December 2005
Abstract A simple detection method based on fluorescence excitation-emission spectroscopy has been developed to quantify the concentration of Ludox colloidal silica nanoparticles having diameters ranging from 7 to 22 nm. The technique works for suspensions of nominally negatively (SM30, LS, and TM40) and positively (CL) charged colloid silica particles. Without pretreatment, the method detection limits are 1.4 mg/L SiO2 for CL (12 nm) and TM40 (22 nm) suspensions and 12 mg/L SiO2 for SM30 (7 nm) and LS (12 nm) suspensions at an excitation wavelength of 308 nm and an emission wavelength of 318 nm. The fluorescence intensities of all the colloidal silica particle suspensions are linear (R2 > 0.98) with concentration in the range of 0–300 mg/L SiO2 . The fluorescence intensity of the negatively charged particle suspensions is constant between pH 3 and 9. The fluorescence intensity of suspensions of particles that are nominally positively charged is constant above pH 5.5, where the particles become negatively charged. © 2005 Elsevier B.V. All rights reserved. Keywords: Ludox colloidal silica particle; Fluorescence spectroscopy; Scattering; Nanoparticle suspension; Zeta potential
1. Introduction For over 30 years, the value of colloidal chemicals and particles in industry has increased due to their extremely high surface area, which makes them ideal for a wide variety of applications such as dispersions, coatings, ceramics, cosmetics, biosensors, colorants, and abrasion-resistant polymers. A large number of studies have dealt with colloidal silica (Ludox silica nanoparticles having diameters ranging from 7 to 100 nm) for a variety of purposes and applications [1–8]. However, a major challenge in working with colloidal silica is the characterization of the suspension properties, particularly the nanoparticle concentration. Numerous means have been used to directly or indirectly determine the size, shape, charge, and/or concentration of colloidal particles using several techniques such as scanning electron microscopy, transmission electron microscopy (TEM), magnetic force microscopy, dynamic light scattering (DLS), atomic force microscopy, X-ray spectrometry, turbidity, ultraviolet spectrometry, and fluorescence
∗
Corresponding author. Tel.: +1 847 491 4265; fax: +1 847 491 3915. E-mail address: [email protected] (R.M. Lueptow).
0927-7757/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2005.11.019
[1,2,4,6–14]. In these studies, various types of Ludox colloidal silica having different sizes and charges have been tested. For instant, a study of the stability of colloidal mixtures containing a poly (vinyl acetate) latex and Ludox AS40 required the measurement of the size distribution of the colloidal dispersion by DLS and TEM [5]. In this study, the distribution was fairly narrow, with a 12 nm mean value for the AS40 particle radius. In another study, the sizes of four Ludox colloidal silica particle samples (SM, HS, AM, TM) having diameters ranging from 7 to 22 nm were measured by laser photoacoustic spectroscopy (LPAS) with an excitation pulse of 308 or 355 nm [6]. The results of this study suggest that scattered light and fluorescent light may also be useful to determine the concentration of colloidal silica particles. Furthermore, studies have shown that Raman scattering spectroscopy is an effective technique to characterize the size distributions of Ludox colloidal silica particle suspensions [6,13]. Only one previous study determined the concentration (40 mg/L) of silica particles, and it was based on turbidity [14]. However, the technique is valid only for particle sizes of 100 nm (±30 nm) or larger. Although other techniques such as dry-weight measurements [15] and dynamic light scattering [8] have been used to directly or indirectly determine the concentration of silica particles
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Y. Yoon, R.M. Lueptow / Colloids and Surfaces A: Physicochem. Eng. Aspects 277 (2006) 107–110
having diameters ranging from 7 to 86 nm, these techniques are time consuming or require difficult analytical techniques. To our knowledge, there are no simple techniques to accurately determine the concentration of colloidal silica suspensions with particles smaller than 30 nm at low concentrations (less than 100 mg/L SiO2 ). Fluorescence spectroscopy has been widely used to characterize specific compounds such as polyaromatic hydrocarbons, aromatic amines, and genotoxic antibiotics [16–18], to monitor biodegradation of organics [19–21], and to determine size and concentration of colloidal silica complexation by a dye [1,22]. Fluorescence is a member of the broad family of luminescence processes in which susceptible atoms and molecules emit light from excited states created by physical (e.g., absorption of light), mechanical (resistance), or chemical mechanisms [23]. Fluorescence provides an extremely sensitive and selective method of qualitative and quantitative analysis [23]. This, along with the success of LPAS in measuring the size of Ludox colloidal silica particles [6], suggests that fluorescence excitation-emission spectroscopy (EES) could provide a quantitative means of measuring colloidal silica concentrations. The objective of this study was to develop a simple quantitative technique for the rapid determination of the concentration of polydisperse Ludox colloidal silica using fluorescence excitation-emission spectroscopy. We focus on positively and negatively charged colloidal silica nanoparticles having diameters ranging from 7 to 22 nm dispersed in water. We consider various pH conditions, because depending on the compounds, pH can have a significant effect on the fluorescence spectrum [23,24]. 2. Measurement technique The technique to measure the concentration of colloidal silica is based on fluorescence EES measurements made using a Hitachi F-2000 fluorescence spectrophotometer (Hitachi Instrument, Ltd., Tokyo, Japan) at an excitation wavelength of 308 nm and an emission wavelength of 318 nm. These fluorescence detection wavelengths were selected based upon fluorescence peaks observed in the three-dimensional excitation-emission matrix (EEM) analysis of the suspensions. The EEM measurements were conducted using a Perkin-Elmer LS-50 (PerkinElmer Life and Analytical Sciences, Inc., Boston, MA, USA) luminescence spectrometer. The excitation source was a 150 W xenon lamp. Excitation and emission slits were maintained at 10 nm and the scan speed was set at 1000 nm/min. The EEM measurements were conducted with the target colloidal silica particles (50 mg/L SiO2 ) at pH 3 and 0.01 M KCl to minimize spectra correction for inner-filter absorbance effects. Excitationemission matrix contour profiles of all silica particles used in this study exhibited two excitation:emission peaks near 200:300 nm and near 308:420 nm. Excitation at 308 nm was used to avoid interference of the Raman water peak that occurs near 200 nm. A range of emission wavelengths were tested for 308 nm excitation. The optimal emission wavelength was 318 nm. A Raman shift value of 1021 cm−1 can be calculated based upon the excitation:emission wavelengths of 308:318 nm, where Raman
Table 1 Characteristics of Ludox colloidal silica according to the manufacturer [15] Ludox type
SM30
LS
CL
TM40
Stabilizing counter ion Particle charge Silica (as SiO2 ), wt% Average diameter (nm) pH @ 25 ◦ C
Sodium Negative 29.8 7 10.0
Sodium Negative 30.2 12 8.2
Chloride Positive 30 12 3.8
Sodium Negative 40.1 22 9.0
scattering is dominant compared to fluorescence absorbance [25,26]. This Raman shift is consistent with that expected for SiO2 [26]. The properties of the aqueous Ludox colloidal silica suspensions (Sigma-Aldrich, St. Louis, MO, USA) that were considered here are summarized in Table 1. The Ludox samples contained silica particles having diameters of 7 nm (SM30), 12 nm (LS and CL), and 22 nm (TM40) and concentrations from 29.8% to 40.1% (w/w). The colloidal silica suspensions were polydispersed based on their polydispersity values ranging from 0.19 to 0.35 [1,22]. According to the manufacturer [15], all the particles were negatively charged at pH levels between 8.2 and 10 with sodium as a stabilizing counter ion except CL, which was positively charged at pH 3.8 in the presence of chloride ions. Stock suspensions at 3000 mg/L SiO2 were prepared by diluting the colloidal silica suspensions from the manufacturer with distilled water. The stock suspensions were subsequently diluted with distilled water to the desired concentrations ranging from 1 to 300 mg/L SiO2 for the calibration. The colloidal silica sample was buffered by adding a 1 M phosphate buffer solution to the sample to provide a 10 mM buffer concentration. Since the fluorescence of the colloidal silica could depend on the pH [23,24], tests were carried out for pH levels between 3 and 10.5. The pH was adjusted to the desired level using very small amounts of 1 M HCl and/or 1 M NaOH solutions. In addition, a 2 M NaCl solution was added to the samples to adjust conductivity to the desired level (300 mS/m) in order to minimize artifacts from different ionic strengths. To characterize the nature of the colloidal silica at the pH levels that were tested, the zeta potential of the silica particles at a concentration of 100 mg/L SiO2 was determined from electrophoretic mobility measurements (Zeta PALS, Brookhaven Instruments Corp., Holtsville, NY, USA). The zeta potentialpH profiles for the silica particles are shown in Fig. 1. Negative zeta potentials were measured over the pH range of 3–10.5 for SM30 (7 nm), LS (12 nm), and TM40 (22 nm). The zeta potential for CL (12 nm) is positive up to a pH of about 5.5 and negative above this pH. The zeta potential becomes more negative with increasing pH and particle size for the (nominally) negatively charged particles (SM30, LS, and TM40). However, the 12 nm (nominally) positively charged particles (CL) have the largest negative zeta potential above the pH 9.5. The measured results confirm the manufacturer’s data that SM30, LS, and TM40 are negative at pH 10, 8.2, and 9.0, and CL is positive at 3.8. The reproducibility for the zeta potential was within 5% (coefficient variance, c.v.) based on triplicates of the measurements.
Y. Yoon, R.M. Lueptow / Colloids and Surfaces A: Physicochem. Eng. Aspects 277 (2006) 107–110
Fig. 1. Dependence of zeta potential of the various colloidal silica particles (100 mg/L SiO2 ) on the pH. () 7 nm (negative, SM30); () 12 nm (negative, LS); (䊉) 12 nm (positive, CL); (♦) 22 nm (negative, TM40). Error bars are smaller than the symbols in most cases.
3. Results and discussion The fluorescence intensities of all four types of colloidal silica particles measured by EES at pH 7 are presented in Fig. 2. All the measurements were performed at room temperature (20 ± 1 ◦ C). Because of instrument-to-instrument variations, the intensities are normalized by the reference intensity, I0 (35 ± 1.3), for a zero concentration of SiO2 particles. Fluorescence intensity increases with increasing particle size for the negatively charged particles. However, 12 nm positively charged particles have the largest fluorescence intensity of all the particles. The fluorescence intensity in all cases is clearly linear (R2 > 0.98) in the range of 0 to 300 mg/L SiO2 . The inset in Fig. 2 shows that the relation is linear even at very low concentrations for 22 nm (TM40) particles, indicating the robustness of the technique. Furthermore, the fluorescence intensity at 300 mg/L SiO2 is nearly 10
Fig. 2. Dependence of fluorescence intensity on the colloidal silica concentration at pH 7. () 7 nm (negative, SM30); () 12 nm (negative, LS); (䊉) 12 nm (positive, CL); (♦) 22 nm (negative, TM40). I is the measured fluorescence intensity at [SiO2 ], and I0 (35 ± 1.3) is the fluorescence intensity at the reference condition (zero SiO2 ). The insert shows the detail at low concentrations for 22 nm particles.
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Fig. 3. Dependence of fluorescence intensity on the 22 nm colloidal silica (TM40) concentration at various pH levels. () pH 3; (♦) pH 7; () pH 10.5.
(SM30), 20 (LS), 60 (TM40), and 170 (CL) times the intensity for zero concentration, indicating the dynamic range of the technique. Method detection limits (MDLs) were determined using a 10 mg/L suspension of 7 nm (SM30) and 12 nm (LS) particles and a 1.0 mg/L suspension of 12 nm (CL) and 22 nm (TM40) particles. The experiments were repeated eight times and the MDL was calculated based upon the standard deviation (c.v. = <5%) of the replicate measurements following the United States Environmental Protection Agency MDL method (Revision 1.1) [27]. MDLs are 1.4 mg/L SiO2 for 12 nm (CL) and 22 nm (TM40) particles and 12 mg/L SiO2 for 7 nm (SM30) and 12 nm (LS) particles. Error bars based on standard deviation calculated from triplicates of fluorescence intensity measurements are smaller than the symbols in the figure. The fluorescence intensities for 22 nm particles measured at various pH levels (3, 7, and 10.5) are presented in Fig. 3 for very low concentrations. The pH of the samples was monitored before and after each fluorescence measurement. All the pH values stayed consistent over the entire range of pH that was measured, even though the phosphate buffer is commonly effective in a pH range of 5.0–8.0 [28]. The differences of the fluorescence intensity are negligible for pH levels of 3 and 7. However, increasing the pH from 7 to 10.5 results in a very significant increase in the fluorescence intensity. The manufacturer of the Ludox colloidal silica recommends that negatively charged silica be used at pH 4–6. Above a pH of approximately 10.5, the 22 nm silica particle suspension becomes increasingly solubilized and the alkali silicate acts like any other soluble salt destabilizing the remaining colloidal silica [15]. In addition, high concentrations of sodium hydroxide also cause aggregation of silica, which can increase particle size, Raman scattering, and fluorescence intensity [15]. The dependence of fluorescence intensities of the colloidal silica suspensions on the pH is shown in Fig. 4 for a single concentration of particles. The fluorescence intensity of the negatively charged particles is constant between pH 3 and 9. However, increasing the pH above 9 results in substantial increases in the fluorescence intensities. This is presumably because disturbance of the charge balance at this high pH level [15] causes
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starting with EEM analysis to determine the appropriate excitation and emission wavelengths could be used to quantify the concentration of nanoparticle suspensions of different materials or particle sizes. Acknowledgements This work was supported by NASA. The authors would also like to thank Dr. Mario Esparza for fluorescence EEM analyses at Arizona State University. References
Fig. 4. Dependence of fluorescence intensity of the various colloidal silica particles (30 mg/L SiO2 ) on the pH. () 7 nm (negative, SM30); () 12 nm (negative, LS); (䊉) 12 nm (positive, CL); (♦) 22 nm (negative, TM40).
the colloidal silica particles to aggregate [29]. The larger aggregates likely result in higher fluorescence much like an increase in particle size causes higher fluorescence intensity [30]. The suspension of positively charged particles shows a different trend for the fluorescence intensity as a function of pH. The fluorescence intensity increases with pH up to a pH of about 5.5 and remains constant above this pH. This may be related to stabilization of the particles in the presence of high concentrations of hydrochloric acid as the pH is decreased. The decrease in size as the particle aggregates break up may result in lower fluorescence intensity at the low pH levels. 4. Conclusions Fluorescence excitation-emission spectroscopy provides a simple and useful tool for quantifying the concentration of Ludox colloidal silica particles having diameters ranging from 7 to 22 nm. Method detection limits were 1.4 mg/L SiO2 for 12 nm (CL) and 22 nm (TM40) particles and 12 mg/L SiO2 for 7 nm (SM30) and 12 nm (LS) particles. Fluorescence intensity increases with increasing particle size. In addition, fluorescence intensity increases with increasing pH above 9 for the negatively charged particles (SM30, LS, and TM40) and decreases with decreasing pH below 5.5 for the positively charged particles (CL). Since the fluorescence intensities depend not only on the colloidal silica concentration but also on the particle size and pH, a separate calibration curve is necessary for each silica sample at specific solution conditions. A similar approach
[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]
X.J. Leng, K. Starchev, J. Buffle, Langmuir 18 (2002) 7602. M. Piech, J.Y. Walz, J. Colloid Interface Sci. 253 (2002) 117. J. Laven, H.N. Stein, J. Colloid Interface Sci. 238 (2001) 8. V. Uricanu, J.R. Eastman, B. Vincent, J. Colloid Interface Sci. 233 (2001) 1. E. Bae, S. Chah, J.H. Yi, J. Colloid Interface Sci. 230 (2000) 367. J.R. Small, N.S. Foster, J.E. Amonette, T. Autrey, Appl. Spectrosc. 54 (2000) 1142. S.R. Kline, E.W. Kaler, J. Colloid Interface Sci. 203 (1998) 392. S. Kluijtmans, J.K.G. Dhont, A.P. Philipse, Langmuir 13 (1997) 4982. M. Rasa, A.P. Philipse, J. Magn. Magn. Mater. 252 (2002) 101. H.H. Park, J. Won, S.G. Oh, Y.S. Kang, J. Membrane Sci. 217 (2003) 285. S. Utsunomiya, R.C. Ewing, Environ. Sci. Technol. 37 (2003) 786. M. Plaschke, J. Romer, J.I. Kim, Environ. Sci. Technol. 36 (2002) 4483. T. Okubo, T. Miyamoto, K. Umemura, K. Kobayashi, Colloid Polym. Sci. 279 (2001) 1236. E.M.V. Hoek, A.S. Kim, M. Elimelech, Environ. Eng. Sci. 19 (2002) 357. GraceDavison, Ludox: Technical Information, Chemical and Physical Properties, http://www.gracedavison.com/products/ludox/techinfo.htm. C. Miege, J. Dugay, M.C. Hennion, J. Chromatogr. A 823 (1998) 219. D. Djozan, M.A. Faraj-Zadeh, Anal. Lett. 31 (1998) 2093. A. Hartmann, A.C. Alder, T. Koller, R.M. Widmer, Environ. Toxicol. Chem. 17 (1998) 377. S. Isaacs, T. Mah, S.K. Maneshin, Water Sci. Technol. 37 (1998) 121. D.M. Reynolds, S.R. Ahmad, Water Res. 31 (1997) 2012. B. Tartakovsky, L.A. Lishman, R.L. Legge, Water Res. 30 (1996) 2941. C.D. Geddes, K. Apperson, D.J. Birch, Dyes Pigments 44 (2000) 69. C.E. White, R.J. Argauer, Fluorescence Analysis: A Practical Approach, Marcel Dekker Inc., New York, 1970. T. Sugimoto, Monodispersed Particles, Elsevier, Amsterdam, 2001. P. Westerhoff, W. Chen, M. Esparza, J. Environ. Qual. 30 (2001) 2037. A. Bertoluzza, C. Fagnano, M.A. Morelli, M. Guglielmi, G. Scarinci, N. Maliavski, J. Raman Spectrosc. 19 (1988) 297. T.D. Behymer, J.S. Bellar, J.S. Ho, W.L. Budde, US Environmental Protection Agency, Supplement II; Revision 1.1, Cincinnati, 1993. D.D. Perrin, Buffers for pH and Metal Ion Control, Chapman and Hall, London, 1974. S.J. Allison, J. Colloid Interface Sci. 277 (2004) 248. Z.H. Zhu, T. Zhu, Z.F. Liu, Nanotechnology 15 (2004) 357.
Concentration of colloidal silica suspensions using fluorescence spectroscopy Yeomin Yoon, Richard M. Lueptow ∗ Department of Mechanical Engineering, Northwestern University, Evanston, IL, 60208, USA Received 21 October 2005; accepted 5 November 2005 Available online 15 December 2005
Abstract A simple detection method based on fluorescence excitation-emission spectroscopy has been developed to quantify the concentration of Ludox colloidal silica nanoparticles having diameters ranging from 7 to 22 nm. The technique works for suspensions of nominally negatively (SM30, LS, and TM40) and positively (CL) charged colloid silica particles. Without pretreatment, the method detection limits are 1.4 mg/L SiO2 for CL (12 nm) and TM40 (22 nm) suspensions and 12 mg/L SiO2 for SM30 (7 nm) and LS (12 nm) suspensions at an excitation wavelength of 308 nm and an emission wavelength of 318 nm. The fluorescence intensities of all the colloidal silica particle suspensions are linear (R2 > 0.98) with concentration in the range of 0–300 mg/L SiO2 . The fluorescence intensity of the negatively charged particle suspensions is constant between pH 3 and 9. The fluorescence intensity of suspensions of particles that are nominally positively charged is constant above pH 5.5, where the particles become negatively charged. © 2005 Elsevier B.V. All rights reserved. Keywords: Ludox colloidal silica particle; Fluorescence spectroscopy; Scattering; Nanoparticle suspension; Zeta potential
1. Introduction For over 30 years, the value of colloidal chemicals and particles in industry has increased due to their extremely high surface area, which makes them ideal for a wide variety of applications such as dispersions, coatings, ceramics, cosmetics, biosensors, colorants, and abrasion-resistant polymers. A large number of studies have dealt with colloidal silica (Ludox silica nanoparticles having diameters ranging from 7 to 100 nm) for a variety of purposes and applications [1–8]. However, a major challenge in working with colloidal silica is the characterization of the suspension properties, particularly the nanoparticle concentration. Numerous means have been used to directly or indirectly determine the size, shape, charge, and/or concentration of colloidal particles using several techniques such as scanning electron microscopy, transmission electron microscopy (TEM), magnetic force microscopy, dynamic light scattering (DLS), atomic force microscopy, X-ray spectrometry, turbidity, ultraviolet spectrometry, and fluorescence
∗
Corresponding author. Tel.: +1 847 491 4265; fax: +1 847 491 3915. E-mail address: [email protected] (R.M. Lueptow).
0927-7757/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2005.11.019
[1,2,4,6–14]. In these studies, various types of Ludox colloidal silica having different sizes and charges have been tested. For instant, a study of the stability of colloidal mixtures containing a poly (vinyl acetate) latex and Ludox AS40 required the measurement of the size distribution of the colloidal dispersion by DLS and TEM [5]. In this study, the distribution was fairly narrow, with a 12 nm mean value for the AS40 particle radius. In another study, the sizes of four Ludox colloidal silica particle samples (SM, HS, AM, TM) having diameters ranging from 7 to 22 nm were measured by laser photoacoustic spectroscopy (LPAS) with an excitation pulse of 308 or 355 nm [6]. The results of this study suggest that scattered light and fluorescent light may also be useful to determine the concentration of colloidal silica particles. Furthermore, studies have shown that Raman scattering spectroscopy is an effective technique to characterize the size distributions of Ludox colloidal silica particle suspensions [6,13]. Only one previous study determined the concentration (40 mg/L) of silica particles, and it was based on turbidity [14]. However, the technique is valid only for particle sizes of 100 nm (±30 nm) or larger. Although other techniques such as dry-weight measurements [15] and dynamic light scattering [8] have been used to directly or indirectly determine the concentration of silica particles
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having diameters ranging from 7 to 86 nm, these techniques are time consuming or require difficult analytical techniques. To our knowledge, there are no simple techniques to accurately determine the concentration of colloidal silica suspensions with particles smaller than 30 nm at low concentrations (less than 100 mg/L SiO2 ). Fluorescence spectroscopy has been widely used to characterize specific compounds such as polyaromatic hydrocarbons, aromatic amines, and genotoxic antibiotics [16–18], to monitor biodegradation of organics [19–21], and to determine size and concentration of colloidal silica complexation by a dye [1,22]. Fluorescence is a member of the broad family of luminescence processes in which susceptible atoms and molecules emit light from excited states created by physical (e.g., absorption of light), mechanical (resistance), or chemical mechanisms [23]. Fluorescence provides an extremely sensitive and selective method of qualitative and quantitative analysis [23]. This, along with the success of LPAS in measuring the size of Ludox colloidal silica particles [6], suggests that fluorescence excitation-emission spectroscopy (EES) could provide a quantitative means of measuring colloidal silica concentrations. The objective of this study was to develop a simple quantitative technique for the rapid determination of the concentration of polydisperse Ludox colloidal silica using fluorescence excitation-emission spectroscopy. We focus on positively and negatively charged colloidal silica nanoparticles having diameters ranging from 7 to 22 nm dispersed in water. We consider various pH conditions, because depending on the compounds, pH can have a significant effect on the fluorescence spectrum [23,24]. 2. Measurement technique The technique to measure the concentration of colloidal silica is based on fluorescence EES measurements made using a Hitachi F-2000 fluorescence spectrophotometer (Hitachi Instrument, Ltd., Tokyo, Japan) at an excitation wavelength of 308 nm and an emission wavelength of 318 nm. These fluorescence detection wavelengths were selected based upon fluorescence peaks observed in the three-dimensional excitation-emission matrix (EEM) analysis of the suspensions. The EEM measurements were conducted using a Perkin-Elmer LS-50 (PerkinElmer Life and Analytical Sciences, Inc., Boston, MA, USA) luminescence spectrometer. The excitation source was a 150 W xenon lamp. Excitation and emission slits were maintained at 10 nm and the scan speed was set at 1000 nm/min. The EEM measurements were conducted with the target colloidal silica particles (50 mg/L SiO2 ) at pH 3 and 0.01 M KCl to minimize spectra correction for inner-filter absorbance effects. Excitationemission matrix contour profiles of all silica particles used in this study exhibited two excitation:emission peaks near 200:300 nm and near 308:420 nm. Excitation at 308 nm was used to avoid interference of the Raman water peak that occurs near 200 nm. A range of emission wavelengths were tested for 308 nm excitation. The optimal emission wavelength was 318 nm. A Raman shift value of 1021 cm−1 can be calculated based upon the excitation:emission wavelengths of 308:318 nm, where Raman
Table 1 Characteristics of Ludox colloidal silica according to the manufacturer [15] Ludox type
SM30
LS
CL
TM40
Stabilizing counter ion Particle charge Silica (as SiO2 ), wt% Average diameter (nm) pH @ 25 ◦ C
Sodium Negative 29.8 7 10.0
Sodium Negative 30.2 12 8.2
Chloride Positive 30 12 3.8
Sodium Negative 40.1 22 9.0
scattering is dominant compared to fluorescence absorbance [25,26]. This Raman shift is consistent with that expected for SiO2 [26]. The properties of the aqueous Ludox colloidal silica suspensions (Sigma-Aldrich, St. Louis, MO, USA) that were considered here are summarized in Table 1. The Ludox samples contained silica particles having diameters of 7 nm (SM30), 12 nm (LS and CL), and 22 nm (TM40) and concentrations from 29.8% to 40.1% (w/w). The colloidal silica suspensions were polydispersed based on their polydispersity values ranging from 0.19 to 0.35 [1,22]. According to the manufacturer [15], all the particles were negatively charged at pH levels between 8.2 and 10 with sodium as a stabilizing counter ion except CL, which was positively charged at pH 3.8 in the presence of chloride ions. Stock suspensions at 3000 mg/L SiO2 were prepared by diluting the colloidal silica suspensions from the manufacturer with distilled water. The stock suspensions were subsequently diluted with distilled water to the desired concentrations ranging from 1 to 300 mg/L SiO2 for the calibration. The colloidal silica sample was buffered by adding a 1 M phosphate buffer solution to the sample to provide a 10 mM buffer concentration. Since the fluorescence of the colloidal silica could depend on the pH [23,24], tests were carried out for pH levels between 3 and 10.5. The pH was adjusted to the desired level using very small amounts of 1 M HCl and/or 1 M NaOH solutions. In addition, a 2 M NaCl solution was added to the samples to adjust conductivity to the desired level (300 mS/m) in order to minimize artifacts from different ionic strengths. To characterize the nature of the colloidal silica at the pH levels that were tested, the zeta potential of the silica particles at a concentration of 100 mg/L SiO2 was determined from electrophoretic mobility measurements (Zeta PALS, Brookhaven Instruments Corp., Holtsville, NY, USA). The zeta potentialpH profiles for the silica particles are shown in Fig. 1. Negative zeta potentials were measured over the pH range of 3–10.5 for SM30 (7 nm), LS (12 nm), and TM40 (22 nm). The zeta potential for CL (12 nm) is positive up to a pH of about 5.5 and negative above this pH. The zeta potential becomes more negative with increasing pH and particle size for the (nominally) negatively charged particles (SM30, LS, and TM40). However, the 12 nm (nominally) positively charged particles (CL) have the largest negative zeta potential above the pH 9.5. The measured results confirm the manufacturer’s data that SM30, LS, and TM40 are negative at pH 10, 8.2, and 9.0, and CL is positive at 3.8. The reproducibility for the zeta potential was within 5% (coefficient variance, c.v.) based on triplicates of the measurements.
Y. Yoon, R.M. Lueptow / Colloids and Surfaces A: Physicochem. Eng. Aspects 277 (2006) 107–110
Fig. 1. Dependence of zeta potential of the various colloidal silica particles (100 mg/L SiO2 ) on the pH. () 7 nm (negative, SM30); () 12 nm (negative, LS); (䊉) 12 nm (positive, CL); (♦) 22 nm (negative, TM40). Error bars are smaller than the symbols in most cases.
3. Results and discussion The fluorescence intensities of all four types of colloidal silica particles measured by EES at pH 7 are presented in Fig. 2. All the measurements were performed at room temperature (20 ± 1 ◦ C). Because of instrument-to-instrument variations, the intensities are normalized by the reference intensity, I0 (35 ± 1.3), for a zero concentration of SiO2 particles. Fluorescence intensity increases with increasing particle size for the negatively charged particles. However, 12 nm positively charged particles have the largest fluorescence intensity of all the particles. The fluorescence intensity in all cases is clearly linear (R2 > 0.98) in the range of 0 to 300 mg/L SiO2 . The inset in Fig. 2 shows that the relation is linear even at very low concentrations for 22 nm (TM40) particles, indicating the robustness of the technique. Furthermore, the fluorescence intensity at 300 mg/L SiO2 is nearly 10
Fig. 2. Dependence of fluorescence intensity on the colloidal silica concentration at pH 7. () 7 nm (negative, SM30); () 12 nm (negative, LS); (䊉) 12 nm (positive, CL); (♦) 22 nm (negative, TM40). I is the measured fluorescence intensity at [SiO2 ], and I0 (35 ± 1.3) is the fluorescence intensity at the reference condition (zero SiO2 ). The insert shows the detail at low concentrations for 22 nm particles.
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Fig. 3. Dependence of fluorescence intensity on the 22 nm colloidal silica (TM40) concentration at various pH levels. () pH 3; (♦) pH 7; () pH 10.5.
(SM30), 20 (LS), 60 (TM40), and 170 (CL) times the intensity for zero concentration, indicating the dynamic range of the technique. Method detection limits (MDLs) were determined using a 10 mg/L suspension of 7 nm (SM30) and 12 nm (LS) particles and a 1.0 mg/L suspension of 12 nm (CL) and 22 nm (TM40) particles. The experiments were repeated eight times and the MDL was calculated based upon the standard deviation (c.v. = <5%) of the replicate measurements following the United States Environmental Protection Agency MDL method (Revision 1.1) [27]. MDLs are 1.4 mg/L SiO2 for 12 nm (CL) and 22 nm (TM40) particles and 12 mg/L SiO2 for 7 nm (SM30) and 12 nm (LS) particles. Error bars based on standard deviation calculated from triplicates of fluorescence intensity measurements are smaller than the symbols in the figure. The fluorescence intensities for 22 nm particles measured at various pH levels (3, 7, and 10.5) are presented in Fig. 3 for very low concentrations. The pH of the samples was monitored before and after each fluorescence measurement. All the pH values stayed consistent over the entire range of pH that was measured, even though the phosphate buffer is commonly effective in a pH range of 5.0–8.0 [28]. The differences of the fluorescence intensity are negligible for pH levels of 3 and 7. However, increasing the pH from 7 to 10.5 results in a very significant increase in the fluorescence intensity. The manufacturer of the Ludox colloidal silica recommends that negatively charged silica be used at pH 4–6. Above a pH of approximately 10.5, the 22 nm silica particle suspension becomes increasingly solubilized and the alkali silicate acts like any other soluble salt destabilizing the remaining colloidal silica [15]. In addition, high concentrations of sodium hydroxide also cause aggregation of silica, which can increase particle size, Raman scattering, and fluorescence intensity [15]. The dependence of fluorescence intensities of the colloidal silica suspensions on the pH is shown in Fig. 4 for a single concentration of particles. The fluorescence intensity of the negatively charged particles is constant between pH 3 and 9. However, increasing the pH above 9 results in substantial increases in the fluorescence intensities. This is presumably because disturbance of the charge balance at this high pH level [15] causes
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starting with EEM analysis to determine the appropriate excitation and emission wavelengths could be used to quantify the concentration of nanoparticle suspensions of different materials or particle sizes. Acknowledgements This work was supported by NASA. The authors would also like to thank Dr. Mario Esparza for fluorescence EEM analyses at Arizona State University. References
Fig. 4. Dependence of fluorescence intensity of the various colloidal silica particles (30 mg/L SiO2 ) on the pH. () 7 nm (negative, SM30); () 12 nm (negative, LS); (䊉) 12 nm (positive, CL); (♦) 22 nm (negative, TM40).
the colloidal silica particles to aggregate [29]. The larger aggregates likely result in higher fluorescence much like an increase in particle size causes higher fluorescence intensity [30]. The suspension of positively charged particles shows a different trend for the fluorescence intensity as a function of pH. The fluorescence intensity increases with pH up to a pH of about 5.5 and remains constant above this pH. This may be related to stabilization of the particles in the presence of high concentrations of hydrochloric acid as the pH is decreased. The decrease in size as the particle aggregates break up may result in lower fluorescence intensity at the low pH levels. 4. Conclusions Fluorescence excitation-emission spectroscopy provides a simple and useful tool for quantifying the concentration of Ludox colloidal silica particles having diameters ranging from 7 to 22 nm. Method detection limits were 1.4 mg/L SiO2 for 12 nm (CL) and 22 nm (TM40) particles and 12 mg/L SiO2 for 7 nm (SM30) and 12 nm (LS) particles. Fluorescence intensity increases with increasing particle size. In addition, fluorescence intensity increases with increasing pH above 9 for the negatively charged particles (SM30, LS, and TM40) and decreases with decreasing pH below 5.5 for the positively charged particles (CL). Since the fluorescence intensities depend not only on the colloidal silica concentration but also on the particle size and pH, a separate calibration curve is necessary for each silica sample at specific solution conditions. A similar approach
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