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Directed fluorescence sensor element for standoff detection of uranium in soil
Sensors and Actuators B 138 (2009) 134–137
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Directed fluorescence sensor element for standoff detection of uranium in soil Dmitry Pestov a , Chien-Cheng Chen a , Jean D. Nelson b,c , John E. Anderson b,c , Gary Tepper a,∗ a
Mechanical Engineering, Virginia Commonwealth University, Richmond, VA, USA US Army Engineer Research and Development Center, Topographic Engineering Center, Alexandria, VA, USA c Fluorescence Spectroscopy Lab, Life Sciences, Virginia Commonwealth University, Richmond, VA, USA b
a r t i c l e
i n f o
Article history: Received 30 September 2008 Received in revised form 10 February 2009 Accepted 20 February 2009 Available online 9 March 2009 Keywords: Uranyl Fluorescence standoff sensor Directed fluorescence Silica gel
a b s t r a c t A passive signal enhancement device for the standoff detection of uranium in soil was developed and tested. The device consists of a spherical ball lens half-coated with a polymer–silica gel composite. The nanoporous silica gel, when placed in contact with moist soil, absorbs water and dissolved uranyl ions and significantly enhances the fluorescence intensity of the uranyl. The ball lens focuses the UV excitation energy to the focal point of the lens located within the silica gel layer and directs the resulting fluorescence signal back towards the excitation source. Our results show that this ‘Directed Fluorescence’ (DF) device can be used to enhance the uranyl fluorescence signal intensity by more than 200 times. Consequently, the maximum standoff detection distance is increased by more than an order of magnitude. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Uranium is a significant soil and water contaminant at sites associated with uranium mining, nuclear fuel production and disposal [1–5]. Uranium contamination also occurs at locations where its compounds have accumulated through leaching by carbonate-rich irrigation waters and with certain natural geologic processes [6–8]. In the top soil layer uranium usually appears as a complex of the uranyl ion UO2 2+ which is in its highest oxidation state (VI+) [9,10]. Because uranyl is water soluble, it is readily transported through most soil matrices and the rate of uranyl migration depends on a variety of parameters including soil porosity and composition, water content and temperature. Uranyl is an optically active molecule exhibiting green fluorescence under UV irradiation [11,12] making it amenable to optical standoff detection methods. Unfortunately, uranyl contaminated soils exhibit relatively weak fluorescence signals because only the very top layer of the soil is interrogated by the optical probe. Nevertheless, it has been shown that the uranyl fluorescence signal is significantly enhanced when the molecule is absorbed within certain natural inorganic matrices [13,14] including porous silica [15]. Matrix enhancement of the uranyl fluorescence intensity depends on a variety of parameters including pH, matrix porosity, water content and temperature and is due primarily to the close association of uranyl with the silica surface, which minimizes quenching by complexation with water molecules [15–17]. A detailed study of the rate
of uranyl diffusion into porous silica matrices as a function of water content and the specific mechanisms of fluorescence enhancement is beyond the scope of this paper and will be reported in a separate publication. The objective of the present study is to demonstrate that a sensor element consisting of an inorganic matrix (i.e. silica gel) coupled to a spherical ball lens can be used to facilitate standoff detection methods and provide an attractive alternative to conventional methods requiring soil collection and analysis. Fig. 1 is an illustration depicting the optical interrogation of uranium-contaminated soil with and without the directed fluorescence device. Without the directed fluorescence device the fluorescence emission intensity is distributed randomly over 4 even if the excitation is made with a parallel laser beam and only a small fraction of the emitted light actually reaches the detector. With the directed fluorescence device, the uranyl fluorescence signal is first chemically enhanced by interactions with the silica gel matrix and then optically directed back towards the excitation source using the UV transparent spherical ball lens, thereby minimizing the amount of geometric signal attenuation. The fluorescence enhancement layer is deposited as a film on one half of the spherical ball lens and consists of nanoporous silica gel particles supported within a poly(vinyl alcohol) polymer binder and is designed to extract and concentrate uranyl ions from the top soil layer and to chemically enhance the fluorescence intensity. 2. Experimental 2.1. Test station for the directed fluorescence (DF) measurement
∗ Corresponding author. Tel.: +1 804 827 4079; fax: +1 804 828 3846. E-mail address: [email protected] (G. Tepper). 0925-4005/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2009.02.049
Fig. 2 is a schematic diagram of the experimental system used to evaluate the effect of standoff distance on the signal intensity.
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same for all experiments and was a circular spot, 6.5 mm in diameter. Prior to each optical measurement the sand was remoistened by adding 0.5 ml of DI water using a pipet and then remixing the sand to ensure a homogeneous distribution. The emission spectrum was then obtained for the bare sand and with each component of the DF sensor element while the sand was still moist. Then, each system (sand plus sensor element) was air dried for 24 h and the peak emission wavelength was measured as a function of standoff distance as described below. 2.2. Materials
Fig. 1. Optical schemes for regular (A) and directed fluorescence system (B). 1: UV excitation source, 2: signal receiver, 3: uranyl fluorescence enhancer, 4: tested soil, 5: glass or polymer transparent bead, covered with the uranyl fluorescence enhancer.
The system is shown with the directed fluorescence sensor element in place and includes a ball lens (D) half-coated with the fluorescence enhancing material (E). The directed fluorescence sensor is placed onto uranyl-contaminated sand (F). UV excitation from a lamp/monochromator source (A) is sent through a beam splitter (C) onto the directed fluorescence sensor and the resulting emission signal is reflected from the beam splitter into a movable fiber optic detector (B) mounted on a track to allow the standoff distance to be varied over a range of approximately 1 m. The beam splitter (C) is used to maintain a zero angle between the excitation and emission beams simulating typical standoff conditions where, at large distances, the angle between the excitation source and the detector is usually small. A QM-3 Quanta-Master luminescence spectrometer with a pulsed excitation source and a Fluorolog-3 spectrofluorometer (JY Horiba) with a steady-state excitation source, both with flexible wave guides, were used for fluorescence excitation and detection. A UV excitation wavelength centered at 280 nm was used to induce the fluorescence. The fluorescence emission spectrum (intensity versus wavelength) was then measured for each component of the directed fluorescence device and the intensity versus distance was measured at the peak emission wavelength. Uranyl contaminated sand was prepared by adding of 210 l of 10 mM uranyl nitrate solution to 5 g sand and 1 ml water. The uranium-contaminated sand was thoroughly mixed and allowed to equilibrate for 2 days prior to optical interrogation. These proportions produce a uranium concentration of about 100 ppm. To obtain comparable results, the area of optical interrogation was the
Silica gel was obtained from Acros Organics with an average pore size of 4 nm, and a particle diameter in the range of 40–60 m. Uranyl nitrate hexahydrate was purchased from the American Master Tech Scientific, at A.C.S. reagent grade and was used without further purification. Poly(vinyl alcohol) Mw 89,000–98,000 was purchased from Sigma–Aldrich and according to the catalogue has about 1% residual acetyl groups. Sea sand was obtained from Fisher Scientific and was used as is. The UV-transparent sapphire ball lenses were obtained form Edmund Optics with a diameter of 6.35 mm. 2.3. DF sensor preparation One gram of poly(vinyl alcohol) was dissolved in 20 ml of DI water with continuous stirring at 60 ◦ C. One milliliter of this solution was diluted with 1 ml DI water and mixed with 1 g of the silica gel. Prior to deposition, the sapphire ball lens was cleaned with ethanol and immersed into a 10% sodium metasilicate solution for 30 min, rinsed with DI water and dried. One half of the cleaned sapphire ball lens was coated with a thin (approximately 0.5 mm) layer of the polymer–silica gel paste and the coating was solidified by drying for 24 h at room temperature. 3. Results and discussion Fig. 3 shows photographs of the directed fluorescence (DF) sensor element placed onto uranyl-contaminated sea sand under both ambient light and under the unfocused irradiation of a Hg hand lamp equipped with glass filters and emitting a multi-lined spectrum with a most intense peak at 254 nm. The broad or unfocused UV irradiation from the hand lamp was applied uniformly to both the sensor and the surrounding sand and the photographs of Fig. 3 confirm that the fluorescence signal from the directed fluorescence sensor element is much stronger than the signal from the bare sand, which appears as a dark background in the photograph. In order to quantify the actual degree of signal enhancement from each component of the sensor element, emission spectra were independently obtained for each of the following conditions: A. B. C. D.
Sand contaminated with 100 ppm uranium (as uranyl nitrate), Uncoated ball lens on the contaminated sand, Silica gel particles on the contaminated sand, Ball lens coated with polymer/silica gel film on contaminated sand, E. Ball lens on silica gel particles on the contaminated sand.
Fig. 2. Direct fluorescence measuring system. A: excitation source (UV wavelength = 280 nm), B: emission detector, C: beam splitter, D: sapphire ball lens, E: uranyl fluorescence enhancer, and F: uranyl contaminated sand.
Fig. 4 is a plot of the uranyl fluorescence emission spectrum for each of the conditions A–E listed above obtained within minutes after adding moisture to the sand according to the procedures described in Section 2.1. We have found that the amount of moisture in the sand can influence the kinetics of uranyl migration into the DF sensor and these scans indicate that the kinetics are relatively fast (i.e. seconds) under the conditions reported here. A more detailed
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D. Pestov et al. / Sensors and Actuators B 138 (2009) 134–137
Fig. 3. Photograph of directed fluorescence placed sensor onto uranyl nitrate contaminated sand. A: under ambient light, B: under 254 nm UV irradiation. The sand contamination is equivalent to 100 ppm of uranium.
investigation on the kinetics of uranyl migration as a function of water concentration are beyond the scope of this paper, but will be reported elsewhere. The regularly spaced peaks in the uranyl emission spectrum appear in each scan and are due to the vibrational modes of the two oxygen atoms about the larger central uranium atom. According to the results of Fig. 4, the bare contaminated sand exhibited the weakest fluorescence signal followed by the uncoated ball lens placed in contact with the contaminated sand. The silica gel particles deposited directly onto the surface of the contaminated sand exhibited the next strongest fluorescence followed by the directed fluorescence device. Finally, the strongest fluorescence signal was obtained by placing the uncoated ball lens onto a layer of silica gel particles deposited onto the surface of the contaminated sand. The ball lens/silica gel particles combination provides a higher degree of signal enhancement in comparison to the directed fluorescence device (ball lens with polymer–silica gel half coating) for two reasons: (1) the presence of the polymer binder dilutes the concentration of the silica gel within the optical interrogation region,
and (2) the silica gel powder layer is thicker than the coating applied to the ball lens. However, the directed fluorescence device, because it integrates the silica gel chemical enhancer with the ball lens optical component as a single unit, can be disbursed onto target soil from the air in a one-step process for subsequent standoff optical interrogation. In this operational scenario, a certain fraction of the dispersed DF devices (i.e. those landing in an orientation with the silica gel half coating in contact with the soil) will be operational. The fraction of operational devices should be at least 50%, but could be higher since the device’s asymmetrical weight distribution should favor the operational orientation. Fig. 5 is a plot of the fluorescence signal intensity at the peak emission wavelength versus distance for conditions A–E above. The data of Fig. 5 was obtained after each system (A–E) was allowed to air dry for 24 h. The relative intensity of each scan A–E is in good agreement with the data of Fig. 4, obtained under moist conditions. This demonstrates that, while moisture is required to facilitate uranyl migration into the DF sensor element, the signal enhancement is retained even after the water is removed. The DF sensor element (Fig. 5D) provides significant (200×) signal enhancement in comparison to the bare sand (Fig. 5A) and Fig. 5 shows that the
Fig. 4. Steady-state fluorescence emission spectra at 280 nm excitation wavelength recorded immediately after sample preparation. A: sand contaminated with 100 ppm uranium (as uranyl nitrate), B: ball lens on the contaminated sand, C: silica gel on the contaminated sand, D: ball lens coated with silica gel on contaminated sand, and E: ball lens over silica gel on the contaminated sand.
Fig. 5. Semi-logarithmic plot of fluorescence intensity versus distance for—A: sand contaminated with 100 ppm uranium (in form of uranyl nitrate), B: ball lens on the contaminated sand, C: silica gel on the contaminated sand, D: ball lens coated with silica gel on contaminated sand, and E: ball lens over silica gel on the contaminated sand.
D. Pestov et al. / Sensors and Actuators B 138 (2009) 134–137
enhancement is maintained over the 1-m range of our test apparatus. For all of the curves in Fig. 5, the intensity decreases in inverse proportionality to the square of the standoff distance as expected. Therefore, for a given minimum detection threshold, a 200-times increase in the signal intensity translates to a 14-times increase in the standoff distance. This implies, for example, that a maximum detection distance of 30 yards on bare soil can be increased to approximately one quarter of a mile using the directed fluorescence device described here. The sapphire ball lens was selected to provide the necessary transparency to the UV excitation source and so that the focal point of the lens would fall within the 0.5-mm thick fluorescence enhancement layer on the ball surface. The focal length depends on the lens diameter and refractive index as given by the following equation [18]: EFL =
nD 4(n − 1)
BFL = EFL −
D 2
(1) (2)
where EFL is effective focal length, BFL is back focal length (measured from back surface of sphere), D is ball lens diameter (6.35 mm), and n is material index of refraction. At the UV excitation wavelength of 280 nm, the refractive index of sapphire is 1.824 and it is 1.774 at the peak fluorescence emission wavelength of 498 nm. Therefore, the back focal length (BFL) is equal to 0.339 mm and 0.543 mm for the excitation and peak emission wavelengths, respectively. Thus, the excitation light is focused to a point within the fluorescence enhancement coating and the resulting fluorescence emission signal originates near the focal point so that it will be directed back towards the excitation source with a small degree of optical divergence. 4. Conclusion We have developed and demonstrated a new directed fluorescence (DF) sensor element for the standoff detection of uranium in soil. Our results show that the DF sensor element can be used to increase the signal intensity by 200-times and the maximum standoff distance by 14-times. The DF sensor is based on the integration of a nanoporous silica gel fluorescence enhancement matrix and a spherical ball lens, which provides additional optical signal enhancement. The DF sensor element described in this paper can be modified for the detection of other fluorescent molecules simply by changing the properties of the coating material. Acknowledgment This work was supported by the US Department of Energy NNSA under contract (DE-FG52-06NA27491). References [1] R.G. Riley, J.M. Zachara, F.J. Wobber, Chemical contaminants on DOE lands and selection of contaminant mixtures for subsurface science research, Pacific Northwest Lab, DOE/ER-0547T, Richland, WA, 1992. [2] D.E. Morris, P.G. Allen, J.M. Berg, C.J. Chisholm-Brause, S.D. Conradson, R.J. Donohoe, N.J. Hess, J.A. Musgrave, C.D. Tait, Speciation of uranium in fernald soils by molecular spectroscopic methods: characterization of untreated soils, Environ. Sci. Technol. 30 (1996) 2322–2331. [3] G. Bernhard, G. Geipel, V. Brendler, H. Nitsche, Speciation of uranium in seepage waters of a mine tailing pile studied by time-resolved laser-induced fluorescence spectroscopy (TRLFS), Radiochim. Acta 74 (1996) 87–91. [4] B.C. Bostick, S. Fendorf, M.O. Barnett, P.M. Jardine, S.C. Brooks, Uranyl surface complexes formed on subsurface media from DOE facilities, Soil Sci. Soc. Am. J. 66 (2002) 99–108.
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[5] J.L. deLemos, B.C. Bostick, A.N. Quicksall, J.D. Landis, C.C. George, N.L. Slagowski, T. Rock, D. Brugge, J. Lewis, J.L. Durant, Rapid dissolution of soluble uranyl phases in arid, mine-impacted catchments near Church Rock, NM, Environ. Sci. Technol. 42 (2008) 3951–3957. [6] M.C. Duff, C. Amrhein, P.M. Bertsch, D.B. Hunter, The chemistry of uranium in evaporation pond sediment in the San Joaquin valley, California, USA, using Xray fluorescence and XANES techniques, Geochim. Cosmochim. Acta 61 (1997) 73–81. [7] M.C. Duff, D.E. Morris, D.B. Hunter, P.M. Bertsch, Spectroscopic characterization of uranium in evaporation basin sediments, Geochim. Cosmochim. Acta 64 (2000) 1535–1550. [8] A.P. Meshik, C.M. Hohenberg, O.V. Pravdivtseva, Record of cycling operation of the natural nuclear reactor in the Oklo/Okelobondo area in Gabon, Phys. Rev. Lett. 93 (2004) 182302. [9] D. Langmuir, Uranium solution–mineral equilibria at low temperatures with applications to sedimentary ore deposits, Geochim. Cosmochim. Acta 42 (1978) 547–569. [10] A. Kowal-Fouchard, R. Drot, E. Simoni, J.J. Ehrhardt, Use of spectroscopic techniques for uranium(VI)/montmorillonite interaction modeling, Environ. Sci. Technol. 38 (2004) 1399–1407. [11] H.-S. Chang, V. Korshin Gregory, Z. Wang, M. Zachara John, Adsorption of uranyl on gibbsite: a time-resolved laser-induced fluorescence spectroscopy study, Environ. Sci. Technol. 40 (2006) 1244–1249. [12] Z. Wang, J.M. Zachara, A.G. Joly, A.S. Del Negro, N.J. Hess, Recent development and application of cryogenic laser-induced time-resolved U(VI) and Tc(V) fluorescence spectroscopy and microscopy in environmental research, in: Abstracts of Papers, 232nd ACS National Meeting, San Francisco, CA, United States, September 10–14, 2006, 2006, NUCL-060. [13] J.P. DeNeufville, A. Kasdan, R.J.L. Chimenti, Selective detection of uranium by laser-induced fluorescence: a potential remote-sensing technique. 1. Optical characteristics of uranyl geologic targets, Appl. Opt. 20 (1981) 1279–1296. [14] A. Kasdan, R.J.L. Chimenti, J.P. DeNeufville, Selective detection of uranium by laser-induced fluorescence: a potential remote-sensing technique. 2. Experimental assessment of the remote sensing of uranyl geologic targets, Appl. Opt. 20 (1981) 1297–1307. [15] M. Lopez, D.J.S. Birch, Uranyl photophysics on colloidal silica: an alternative luminescence-enhancing medium for uranyl assay, Analyst 121 (1996) 905–908. [16] Y.D. Glinka, T.B. Krak, Luminescence spectra of uranyl ions adsorbed on disperse SiO2 surfaces, Phys. Rev. B: Condens. Matter 52 (1995) R14985–14995. [17] U. Gabriel, L. Charlet, C.W. Schlapfer, J.C. Vial, A. Brachmann, G. Geipel, Uranyl surface speciation on silica particles studied by time-resolved laser-induced fluorescence spectroscopy, J. Colloid Interface Sci. 239 (2001) 358–368. [18] J.J. Rennilson, in: C. DeCusatis (Ed.), Handbook of Applied Photometry, AIP Press; Optical Society of America, Washington, DC, 1997, p. 300.
Biographies Dmitry Pestov is a research associate in mechanical engineering at Virginia Commonwealth University in Richmond. He obtained his MS (chemical engineering) and later PhD (organic chemistry) from the Saint-Petersburg State Institute of Technology, Russia. His main research interests are on molecular imprinting, materials for chemical sensors, SAW sensors and fluorescence-based chemical sensors. Chien-Cheng Chen is a PhD student in the department of Mechanical Engineering Virginia Commonwealth University. He received the MS degree in National Chung Cheng University, Taiwan R.O.C., 1999. His current fields of interest are optical detecting sensors, fluorescence spectral analysis, and nano-materials for chemical sensors. Jean Nelson is a research scientist for the US Army ERDC Topographic Engineering Center. She obtained her MS (Environmental Studies) and is currently a PhD student (Integrative Life Sciences) at Virginia Commonwealth University in Richmond. Her research interests include fluorescence spectroscopy and improving remote detection of chemical, biological and radiological targets of interest through knowledge of key environmental parameters. John Anderson is a senior research scientist for the US Army ERDC Topographic Engineering Center. He obtained his doctorate in Environmental Biology concentrating in biological spectroscopy and remote sensing from George Mason University in Fairfax, Virginia near Washington, DC. His research interests include passive reflectance and active, laser-induced luminescence of optical reporters for wide-area hazardous materials detection. Gary Tepper is a professor of mechanical engineering and Director of the Microsensor and Radiation Detector Research Laboratories at Virginia Commonwealth University in Richmond. He obtained his PhD from the University of California in San Diego in engineering sciences (engineering physics) and his undergraduate degree from Penn State.
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Directed fluorescence sensor element for standoff detection of uranium in soil Dmitry Pestov a , Chien-Cheng Chen a , Jean D. Nelson b,c , John E. Anderson b,c , Gary Tepper a,∗ a
Mechanical Engineering, Virginia Commonwealth University, Richmond, VA, USA US Army Engineer Research and Development Center, Topographic Engineering Center, Alexandria, VA, USA c Fluorescence Spectroscopy Lab, Life Sciences, Virginia Commonwealth University, Richmond, VA, USA b
a r t i c l e
i n f o
Article history: Received 30 September 2008 Received in revised form 10 February 2009 Accepted 20 February 2009 Available online 9 March 2009 Keywords: Uranyl Fluorescence standoff sensor Directed fluorescence Silica gel
a b s t r a c t A passive signal enhancement device for the standoff detection of uranium in soil was developed and tested. The device consists of a spherical ball lens half-coated with a polymer–silica gel composite. The nanoporous silica gel, when placed in contact with moist soil, absorbs water and dissolved uranyl ions and significantly enhances the fluorescence intensity of the uranyl. The ball lens focuses the UV excitation energy to the focal point of the lens located within the silica gel layer and directs the resulting fluorescence signal back towards the excitation source. Our results show that this ‘Directed Fluorescence’ (DF) device can be used to enhance the uranyl fluorescence signal intensity by more than 200 times. Consequently, the maximum standoff detection distance is increased by more than an order of magnitude. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Uranium is a significant soil and water contaminant at sites associated with uranium mining, nuclear fuel production and disposal [1–5]. Uranium contamination also occurs at locations where its compounds have accumulated through leaching by carbonate-rich irrigation waters and with certain natural geologic processes [6–8]. In the top soil layer uranium usually appears as a complex of the uranyl ion UO2 2+ which is in its highest oxidation state (VI+) [9,10]. Because uranyl is water soluble, it is readily transported through most soil matrices and the rate of uranyl migration depends on a variety of parameters including soil porosity and composition, water content and temperature. Uranyl is an optically active molecule exhibiting green fluorescence under UV irradiation [11,12] making it amenable to optical standoff detection methods. Unfortunately, uranyl contaminated soils exhibit relatively weak fluorescence signals because only the very top layer of the soil is interrogated by the optical probe. Nevertheless, it has been shown that the uranyl fluorescence signal is significantly enhanced when the molecule is absorbed within certain natural inorganic matrices [13,14] including porous silica [15]. Matrix enhancement of the uranyl fluorescence intensity depends on a variety of parameters including pH, matrix porosity, water content and temperature and is due primarily to the close association of uranyl with the silica surface, which minimizes quenching by complexation with water molecules [15–17]. A detailed study of the rate
of uranyl diffusion into porous silica matrices as a function of water content and the specific mechanisms of fluorescence enhancement is beyond the scope of this paper and will be reported in a separate publication. The objective of the present study is to demonstrate that a sensor element consisting of an inorganic matrix (i.e. silica gel) coupled to a spherical ball lens can be used to facilitate standoff detection methods and provide an attractive alternative to conventional methods requiring soil collection and analysis. Fig. 1 is an illustration depicting the optical interrogation of uranium-contaminated soil with and without the directed fluorescence device. Without the directed fluorescence device the fluorescence emission intensity is distributed randomly over 4 even if the excitation is made with a parallel laser beam and only a small fraction of the emitted light actually reaches the detector. With the directed fluorescence device, the uranyl fluorescence signal is first chemically enhanced by interactions with the silica gel matrix and then optically directed back towards the excitation source using the UV transparent spherical ball lens, thereby minimizing the amount of geometric signal attenuation. The fluorescence enhancement layer is deposited as a film on one half of the spherical ball lens and consists of nanoporous silica gel particles supported within a poly(vinyl alcohol) polymer binder and is designed to extract and concentrate uranyl ions from the top soil layer and to chemically enhance the fluorescence intensity. 2. Experimental 2.1. Test station for the directed fluorescence (DF) measurement
∗ Corresponding author. Tel.: +1 804 827 4079; fax: +1 804 828 3846. E-mail address: [email protected] (G. Tepper). 0925-4005/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2009.02.049
Fig. 2 is a schematic diagram of the experimental system used to evaluate the effect of standoff distance on the signal intensity.
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same for all experiments and was a circular spot, 6.5 mm in diameter. Prior to each optical measurement the sand was remoistened by adding 0.5 ml of DI water using a pipet and then remixing the sand to ensure a homogeneous distribution. The emission spectrum was then obtained for the bare sand and with each component of the DF sensor element while the sand was still moist. Then, each system (sand plus sensor element) was air dried for 24 h and the peak emission wavelength was measured as a function of standoff distance as described below. 2.2. Materials
Fig. 1. Optical schemes for regular (A) and directed fluorescence system (B). 1: UV excitation source, 2: signal receiver, 3: uranyl fluorescence enhancer, 4: tested soil, 5: glass or polymer transparent bead, covered with the uranyl fluorescence enhancer.
The system is shown with the directed fluorescence sensor element in place and includes a ball lens (D) half-coated with the fluorescence enhancing material (E). The directed fluorescence sensor is placed onto uranyl-contaminated sand (F). UV excitation from a lamp/monochromator source (A) is sent through a beam splitter (C) onto the directed fluorescence sensor and the resulting emission signal is reflected from the beam splitter into a movable fiber optic detector (B) mounted on a track to allow the standoff distance to be varied over a range of approximately 1 m. The beam splitter (C) is used to maintain a zero angle between the excitation and emission beams simulating typical standoff conditions where, at large distances, the angle between the excitation source and the detector is usually small. A QM-3 Quanta-Master luminescence spectrometer with a pulsed excitation source and a Fluorolog-3 spectrofluorometer (JY Horiba) with a steady-state excitation source, both with flexible wave guides, were used for fluorescence excitation and detection. A UV excitation wavelength centered at 280 nm was used to induce the fluorescence. The fluorescence emission spectrum (intensity versus wavelength) was then measured for each component of the directed fluorescence device and the intensity versus distance was measured at the peak emission wavelength. Uranyl contaminated sand was prepared by adding of 210 l of 10 mM uranyl nitrate solution to 5 g sand and 1 ml water. The uranium-contaminated sand was thoroughly mixed and allowed to equilibrate for 2 days prior to optical interrogation. These proportions produce a uranium concentration of about 100 ppm. To obtain comparable results, the area of optical interrogation was the
Silica gel was obtained from Acros Organics with an average pore size of 4 nm, and a particle diameter in the range of 40–60 m. Uranyl nitrate hexahydrate was purchased from the American Master Tech Scientific, at A.C.S. reagent grade and was used without further purification. Poly(vinyl alcohol) Mw 89,000–98,000 was purchased from Sigma–Aldrich and according to the catalogue has about 1% residual acetyl groups. Sea sand was obtained from Fisher Scientific and was used as is. The UV-transparent sapphire ball lenses were obtained form Edmund Optics with a diameter of 6.35 mm. 2.3. DF sensor preparation One gram of poly(vinyl alcohol) was dissolved in 20 ml of DI water with continuous stirring at 60 ◦ C. One milliliter of this solution was diluted with 1 ml DI water and mixed with 1 g of the silica gel. Prior to deposition, the sapphire ball lens was cleaned with ethanol and immersed into a 10% sodium metasilicate solution for 30 min, rinsed with DI water and dried. One half of the cleaned sapphire ball lens was coated with a thin (approximately 0.5 mm) layer of the polymer–silica gel paste and the coating was solidified by drying for 24 h at room temperature. 3. Results and discussion Fig. 3 shows photographs of the directed fluorescence (DF) sensor element placed onto uranyl-contaminated sea sand under both ambient light and under the unfocused irradiation of a Hg hand lamp equipped with glass filters and emitting a multi-lined spectrum with a most intense peak at 254 nm. The broad or unfocused UV irradiation from the hand lamp was applied uniformly to both the sensor and the surrounding sand and the photographs of Fig. 3 confirm that the fluorescence signal from the directed fluorescence sensor element is much stronger than the signal from the bare sand, which appears as a dark background in the photograph. In order to quantify the actual degree of signal enhancement from each component of the sensor element, emission spectra were independently obtained for each of the following conditions: A. B. C. D.
Sand contaminated with 100 ppm uranium (as uranyl nitrate), Uncoated ball lens on the contaminated sand, Silica gel particles on the contaminated sand, Ball lens coated with polymer/silica gel film on contaminated sand, E. Ball lens on silica gel particles on the contaminated sand.
Fig. 2. Direct fluorescence measuring system. A: excitation source (UV wavelength = 280 nm), B: emission detector, C: beam splitter, D: sapphire ball lens, E: uranyl fluorescence enhancer, and F: uranyl contaminated sand.
Fig. 4 is a plot of the uranyl fluorescence emission spectrum for each of the conditions A–E listed above obtained within minutes after adding moisture to the sand according to the procedures described in Section 2.1. We have found that the amount of moisture in the sand can influence the kinetics of uranyl migration into the DF sensor and these scans indicate that the kinetics are relatively fast (i.e. seconds) under the conditions reported here. A more detailed
136
D. Pestov et al. / Sensors and Actuators B 138 (2009) 134–137
Fig. 3. Photograph of directed fluorescence placed sensor onto uranyl nitrate contaminated sand. A: under ambient light, B: under 254 nm UV irradiation. The sand contamination is equivalent to 100 ppm of uranium.
investigation on the kinetics of uranyl migration as a function of water concentration are beyond the scope of this paper, but will be reported elsewhere. The regularly spaced peaks in the uranyl emission spectrum appear in each scan and are due to the vibrational modes of the two oxygen atoms about the larger central uranium atom. According to the results of Fig. 4, the bare contaminated sand exhibited the weakest fluorescence signal followed by the uncoated ball lens placed in contact with the contaminated sand. The silica gel particles deposited directly onto the surface of the contaminated sand exhibited the next strongest fluorescence followed by the directed fluorescence device. Finally, the strongest fluorescence signal was obtained by placing the uncoated ball lens onto a layer of silica gel particles deposited onto the surface of the contaminated sand. The ball lens/silica gel particles combination provides a higher degree of signal enhancement in comparison to the directed fluorescence device (ball lens with polymer–silica gel half coating) for two reasons: (1) the presence of the polymer binder dilutes the concentration of the silica gel within the optical interrogation region,
and (2) the silica gel powder layer is thicker than the coating applied to the ball lens. However, the directed fluorescence device, because it integrates the silica gel chemical enhancer with the ball lens optical component as a single unit, can be disbursed onto target soil from the air in a one-step process for subsequent standoff optical interrogation. In this operational scenario, a certain fraction of the dispersed DF devices (i.e. those landing in an orientation with the silica gel half coating in contact with the soil) will be operational. The fraction of operational devices should be at least 50%, but could be higher since the device’s asymmetrical weight distribution should favor the operational orientation. Fig. 5 is a plot of the fluorescence signal intensity at the peak emission wavelength versus distance for conditions A–E above. The data of Fig. 5 was obtained after each system (A–E) was allowed to air dry for 24 h. The relative intensity of each scan A–E is in good agreement with the data of Fig. 4, obtained under moist conditions. This demonstrates that, while moisture is required to facilitate uranyl migration into the DF sensor element, the signal enhancement is retained even after the water is removed. The DF sensor element (Fig. 5D) provides significant (200×) signal enhancement in comparison to the bare sand (Fig. 5A) and Fig. 5 shows that the
Fig. 4. Steady-state fluorescence emission spectra at 280 nm excitation wavelength recorded immediately after sample preparation. A: sand contaminated with 100 ppm uranium (as uranyl nitrate), B: ball lens on the contaminated sand, C: silica gel on the contaminated sand, D: ball lens coated with silica gel on contaminated sand, and E: ball lens over silica gel on the contaminated sand.
Fig. 5. Semi-logarithmic plot of fluorescence intensity versus distance for—A: sand contaminated with 100 ppm uranium (in form of uranyl nitrate), B: ball lens on the contaminated sand, C: silica gel on the contaminated sand, D: ball lens coated with silica gel on contaminated sand, and E: ball lens over silica gel on the contaminated sand.
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enhancement is maintained over the 1-m range of our test apparatus. For all of the curves in Fig. 5, the intensity decreases in inverse proportionality to the square of the standoff distance as expected. Therefore, for a given minimum detection threshold, a 200-times increase in the signal intensity translates to a 14-times increase in the standoff distance. This implies, for example, that a maximum detection distance of 30 yards on bare soil can be increased to approximately one quarter of a mile using the directed fluorescence device described here. The sapphire ball lens was selected to provide the necessary transparency to the UV excitation source and so that the focal point of the lens would fall within the 0.5-mm thick fluorescence enhancement layer on the ball surface. The focal length depends on the lens diameter and refractive index as given by the following equation [18]: EFL =
nD 4(n − 1)
BFL = EFL −
D 2
(1) (2)
where EFL is effective focal length, BFL is back focal length (measured from back surface of sphere), D is ball lens diameter (6.35 mm), and n is material index of refraction. At the UV excitation wavelength of 280 nm, the refractive index of sapphire is 1.824 and it is 1.774 at the peak fluorescence emission wavelength of 498 nm. Therefore, the back focal length (BFL) is equal to 0.339 mm and 0.543 mm for the excitation and peak emission wavelengths, respectively. Thus, the excitation light is focused to a point within the fluorescence enhancement coating and the resulting fluorescence emission signal originates near the focal point so that it will be directed back towards the excitation source with a small degree of optical divergence. 4. Conclusion We have developed and demonstrated a new directed fluorescence (DF) sensor element for the standoff detection of uranium in soil. Our results show that the DF sensor element can be used to increase the signal intensity by 200-times and the maximum standoff distance by 14-times. The DF sensor is based on the integration of a nanoporous silica gel fluorescence enhancement matrix and a spherical ball lens, which provides additional optical signal enhancement. The DF sensor element described in this paper can be modified for the detection of other fluorescent molecules simply by changing the properties of the coating material. Acknowledgment This work was supported by the US Department of Energy NNSA under contract (DE-FG52-06NA27491). References [1] R.G. Riley, J.M. Zachara, F.J. Wobber, Chemical contaminants on DOE lands and selection of contaminant mixtures for subsurface science research, Pacific Northwest Lab, DOE/ER-0547T, Richland, WA, 1992. [2] D.E. Morris, P.G. Allen, J.M. Berg, C.J. Chisholm-Brause, S.D. Conradson, R.J. Donohoe, N.J. Hess, J.A. Musgrave, C.D. Tait, Speciation of uranium in fernald soils by molecular spectroscopic methods: characterization of untreated soils, Environ. Sci. Technol. 30 (1996) 2322–2331. [3] G. Bernhard, G. Geipel, V. Brendler, H. Nitsche, Speciation of uranium in seepage waters of a mine tailing pile studied by time-resolved laser-induced fluorescence spectroscopy (TRLFS), Radiochim. Acta 74 (1996) 87–91. [4] B.C. Bostick, S. Fendorf, M.O. Barnett, P.M. Jardine, S.C. Brooks, Uranyl surface complexes formed on subsurface media from DOE facilities, Soil Sci. Soc. Am. J. 66 (2002) 99–108.
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Biographies Dmitry Pestov is a research associate in mechanical engineering at Virginia Commonwealth University in Richmond. He obtained his MS (chemical engineering) and later PhD (organic chemistry) from the Saint-Petersburg State Institute of Technology, Russia. His main research interests are on molecular imprinting, materials for chemical sensors, SAW sensors and fluorescence-based chemical sensors. Chien-Cheng Chen is a PhD student in the department of Mechanical Engineering Virginia Commonwealth University. He received the MS degree in National Chung Cheng University, Taiwan R.O.C., 1999. His current fields of interest are optical detecting sensors, fluorescence spectral analysis, and nano-materials for chemical sensors. Jean Nelson is a research scientist for the US Army ERDC Topographic Engineering Center. She obtained her MS (Environmental Studies) and is currently a PhD student (Integrative Life Sciences) at Virginia Commonwealth University in Richmond. Her research interests include fluorescence spectroscopy and improving remote detection of chemical, biological and radiological targets of interest through knowledge of key environmental parameters. John Anderson is a senior research scientist for the US Army ERDC Topographic Engineering Center. He obtained his doctorate in Environmental Biology concentrating in biological spectroscopy and remote sensing from George Mason University in Fairfax, Virginia near Washington, DC. His research interests include passive reflectance and active, laser-induced luminescence of optical reporters for wide-area hazardous materials detection. Gary Tepper is a professor of mechanical engineering and Director of the Microsensor and Radiation Detector Research Laboratories at Virginia Commonwealth University in Richmond. He obtained his PhD from the University of California in San Diego in engineering sciences (engineering physics) and his undergraduate degree from Penn State.