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Trace analysis of oil-in-water (O/W) has wide applications in life science, industry and environmental monitoring (such as oil spilling). In this paper, with the aid of surfactant, diesel was dispersed in water as O/W emulsion, which can be detected by using visible LED and metal-waveguide-capillary (MWC). Due to the enhancement of optical-path and related light-droplet interaction in MWC, detecting diesel of a concentration as low as 2.14 ng/ml was realized with a 7cm long MWC. The detection limit was improved 125 fold compared with that of commercial spectrophotometer with 1 cm-cuvette. The detecting system features compact, low cost and high sensitivity.
© 2016 Optical Society of America
1. Introduction
Oil and water are the most common liquid materials, which are indispensable for our daily life. Trace analysis of oil-in-water (O/W) has wide applications in life science, industry and environmental monitoring (such as petroleum spilling would cause ocean pollution [1]). Infrared/ultraviolet spectrophotometry [2–7] and liquid chromatography [8, 9] are normally employed for detecting oil with a detection limit of 30-90 ng/mL [5, 7] and 52 ng/g [8], respectively. However, these instruments are expensive and bulky, and noxious organic solvents (such as methylene chloride) are required to dissolve the oil [8]. For spectrophotometer, the instrument can be miniaturized if LED was employed as light source. But, the infrared/ultraviolet LED are expensive and immature compared with visible LED [3].
Meanwhile, O/W emulsion, which enables uniform dispersion of the oil micro-droplet in water, can simplify the sampling process without the use of organic solvents [10–13]. Moreover, there exists a linear relationship between the oil content and optical turbidity, which can be measured at visible wavelength. However, the corresponding detection limit of oil concentration is only on the level of ~10 μg/ml (with Hach DR/4000 spectrophotometer) [14], which needs to be improved dramatically for trace analysis applications.
The turbidity, which results from light-scattering at the surface of oil droplet, can also induce light propagation loss and can be considered as a kind of absorbance. According to previous report, by using metal-waveguide-capillary (MWC), the optical-path can be dramatically enhanced for ultra-sensitive absorbance detection [15]. Thus, for detecting O/W emulsion, the detection limit could be greatly improved if MWC was employed.
In this paper, with the aid of surfactant, diesel (a kind of fuel oil) was dispersed in water as O/W emulsion. Absorbance of the emulsion was measured by using a 505nm visible LED and a 7cm long MWC. Owing to the enhancement of optical-path and light-droplet interaction in MWC, a detection limit of 2.14ng/ml was realized, which is improved nearly 10 fold compared with that of conventional infrared/ultraviolet spectrophotometry and chromatography detection. The detecting system features compact, low cost and high sensitivity.
2. Experimental section
2.1 Chemicals and reagents
Sodium-dodecyl-benzene-sulfonate (SDBS) surfactant of analytical grade was purchased from Tianjin GuangFu Fine Chemical Research Institute. The diesel was provided by the National Marine Environmental Monitoring Center of China. The density of the diesel is 0.84 g/ml.
2.2 Apparatus
As shown in Fig. 1, the MWC-based photometer consists of a 7cm long MWC (1.7 mm i.d., 3.18 mm o.d.), a 505 nm LED (Thorlabs M505F1), a photodetector (Thorlabs PDB450C), and two T-connectors used for optical coupling and fluid inlet/outlet. A three-way valve connected to the inlet pipe is used to switch the inflow samples. The LED light-beam can be easily and efficiently coupled into the MWC through the T-connector.
For comparison, a commercial UV-Visible spectrophotometer (Agilent Technologies Cary 300 Series, equipped with 1.0 cm cuvette cell) was employed to measure the absorbance of the liquid samples. Two pipettes (Finnpipette 1-10 ml and 0~50 μl, Thermo Fisher Scientific Inc.) were used for pipetting solutions.
2.3 Procedures
Three kinds of solutions were prepared, i.e., SDBS solution, diesel-emulsions, and diesel-solutions. The SDBS solution (0.1 wt.%) was prepared by dissolving 0.5g SDBS powder in 500 ml deionized (DI) water. The diesel-emulsions (sample E1-E9) with concentrations ranging from 8.4 × 10−4 to 2.14 × 10−9 g/ml (as listed in Table 1) were prepared by dispersing the diesel in the SDBS solution via ultrasonic vibration. The diesel-solutions (sample S1-S11) with concentrations ranging from 1.34 × 10−3 to 1.38 × 10−10g/ml (listed in Table 2) were prepared by dissolving the diesel in the ethanol.
Table 1. Measurement Results of Diesel-emulsion Samples with Different Concentrations.
Table 2. Diesel-solutions Samples with Different Concentrations.
As listed in Table 1, sample E1 was prepared by mixing 0.1ml diesel with 100 ml SDBS solution (0.1 wt.%), then the solution was stirred (1000 rpm) under the ultrasonic with power and frequency of 120W and 40KHz, respectively, for 3 hours. Then sample E2-E9 were prepared by using successive dilution method [15]. As listed in Table 2, sample S0 was prepared by mixing 0.8 ml diesel with 99.2 ml ethanol. Then sample S1-S11 were prepared by using successive dilution method [15].
The light-beam and the liquid sample were introduced into the MWC via the T-connector, and the beam transmitting through the MWC was received by the photodetector. The diesel-emulsion (as colored-sample) or the SDBS solution (as blank-sample) was introduced alternately into the MWC via the three-way valve. According to Beer’s law, the absorbance of colored-sample can be calculated via Eqs. (1) [16].
Where Vcolor and Vblank are output signals of the photodetector when the colored- and blank-samples are introduced into the MWC, respectively, and Vdark is the background signal of the photodetector when the LED is turned off. The output signal variation ΔV = Vcolor – Vblank can be measured by switching the sample.
For comparison, the diesel-emulsions and diesel-solutions were also measured by using the commercial spectrophotometer.
3. Results and discussions
3.1 Measured by using commercial spectrophotometer
Figure 2 shows the optical photograph of the diesel-emulsion samples (from E1 to E9) with diesel concentrations (as listed in Table 1) ranging from 8.4 × 10−4 (on right) to 2.14 × 10−9 g/ml (on left). The emulsion becomes opaque with increasing diesel concentration, and the microscopy image of sample E1 was shown in Fig. 3. In contrast, without adding SDBS, the diesel just floats on the water as thin film (inset of Fig. 3).
Figure 4 shows the measured absorbance Acuvette of the diesel-emulsions (sample E1~E9) by using the commercial spectrophotometer. As shown in the figure, the detection limit can reach a concentration of 2.69 × 10−7 g/ml (sample E6), because the absorbance curves of sample E7, E8 and E9 cannot be discriminated from each other in the wavelength range of 200nm-800nm. The values of Acuvette at 505 nm (as listed in Table 1) was obtained by regarding the curves of sample E7, E8, or E9 (inset of Fig. 4) as the baseline.
In comparison with the diesel-emulsions, the diesel-solutions (sample S1-S11) were also measured in conventional manner by using the spectrophotometer. As shown in Fig. 5, at 223nm (the ultraviolet absorption peak), the detection limit can reach a concentration of 1.72 × 10−8 g/ml (S8), which is consistent with previous report [4, 6]. While, at visible wavelength (400nm ~780nm), there is almost no absorption. Thus, via emulsifying, the absorption wavelength can be extended from ultraviolet to visible wavelength, which enables the detecting of diesel with visible LED.
3.2 Measured by using the MWC and visible LED
The diesel-emulsions were measured by using the MWC and 505nm LED (Fig. 1). The measurement results of sample E1, E4, E8, and E9 are shown in Figs. 6(a)-(d), respectively, with good repeatability and stability. The time, when switching takes place between the colored-samples (the diesel-emulsion) and blank-samples (the SDBS solution), is marked by arrow “→” in the figures. It is clear that the output voltage increases rapidly when switching from colored- to blank-sample, and it decreases vice versa. By using Eqs. (1), the absorbance AMWC can be calculated with the measured Vcolor, Vblank and constant Vdark.(− 0.296 μV). Measurement results of all samples were summed up in Table 1 (results of sample E2-E3 and E5-E7 were not shown).
Fig. 6 Measurement results of sample (a) E1, (b) E4, (c) E8 and (d) E9 by using the MWC and visible LED.
For sample E9 [Fig. 6(d)], the measured ΔV is only 0.063μV, which is nearly 3 times of the noise value (0.02 μV). Smaller ΔV is hard to be discriminated from the noise. Thus, the detection limit reaches a concentration of 2.14 × 10−9 g/ml (sample E9). It can be concluded that, for the diesel-emulsion detection at visible 505nm, a 125-fold improvement on detection limit was realized compared with that of commercial spectrophotometer (Fig. 4). Moreover, in comparison with the conventional ultraviolet detection of diesel-solution at 223nm by using the commercial spectrophotometer (Fig. 5), a nearly ten-fold improvement on detection limit was realized with the MWC-based visible-light detection.
4. Analysis and discussion
The measured absorbance-concentration relationship of diesel-emulsions (as listed in Table 1) was plotted in Fig. 7. For the commercial spectrophotometry measurement, the absorbance is proportional to the diesel concentration due to the constant optical-path of 1.0 cm cuvette. While for MWC-based measurement, nonlinear enhancement in absorbance is observed at low diesel concentrations (8 × 10−6 ~2 × 10−9 g/ml), due to the nonlinear enhancement on optical-path in MWC [15]
According to Beer’s law, the absorbance is proportional to the optical-path, so the absorbance-enhancement-factor AEF (defined as AEF = AMWC/Acuvette at the same diesel concentration) is the ratio between the optical-path of the MWC and cuvette. As shown in Fig. 7, the AEF has a value of 4.0 at high diesel concentration (sample E2-E4), and it increases to a value of 110 at low concentration (sample E9) by extrapolating the curve of cuvette-based measurement.
As shown in Fig. 8(a), the incident light-beam was scattered at the droplet surface, due to the refractive index difference between the oil droplet (1.46) and surrounding water (1.33). The light scattering could induce light propagation loss as well as the deviation of propagating direction.
As for low diesel concentration (sample E4 - E9), the oil droplet only occupies a very small volume ratio in the emulsion (8 × 10−6 ~2.56 × 10−9). Thus, in cuvette [Fig. 8(a)], most of light-beam propagates freely without being scattered, which limits the detection sensitivity. According to our previous work [15], by employing MWC, the light-beam can be confined inside the metal capillary regardless of the incident-angle, and the light scattered by the rippled metal sidewall would bounce between the metal sidewalls as zigzag-light, whose optical-path was greatly increased for enhancement of light-droplet interaction [Fig. 8(b)]. Thus, an AEF as large as 110 can be obtained by using MWC.
For higher diesel concentration (sample E2~E4), the oil droplet becomes denser (Fig. 3), which promotes the light-droplet interaction and increases the scattering-induced-absorbance. Thus, the straight-light, which propagates through the MWC without bouncing between the metal sidewall, also has large probability of being scattered. In comparison, the zigzag-light would be highly attenuated, due to the increased absorbance and its much longer optical-path. Thus, the straight-light would play more important role in absorbance detection, and the AEF decreases to a value of 4.0 (sample E2-E4). Moreover, for sample E1, the absorbance became saturated and AEF decreases to ~1.0.
It should be noted that for straight-light, the predicted value of AEF is 7.0, because the ratio between the optical-path of the MWC (7.0 cm long) and the cuvette (1.0 cm thick) is 7.0. Thus, AEF less than 7.0 implies that the absorbance was underestimated. As shown in Fig. 8(a), the droplet (functioned as a lens) would make light dispersive with a spread angle of ~20 degree, so in cuvette only a central portion of light was received by the photodetector. In comparison, confining of the light in MWC enables more light received by the photodetector [Fig. 8(b)], which would result in the underestimating of absorbance.
Besides diesel oil, other kind of oils can also be detected, only if the oil-emulsion can be prepared by using specific surfactant. As for distinguish between different oil types, it can be realized by using Mid-Infrared light source with tunable wavelength [1, 17]
It should be noted that precise analysis of light-scattering of oil droplets should be carried out by using Mie-theory rather than geometrical optics. However, for the droplets with diameters (1~5μm) and refractive index (1.46) in our case, the agreement between geometrical optics and Mie-thoery calculations is reasonably good [18]. Thus, for simplicity, geometrical optics was used for analysis of light scattering.
5. Conclusion
A compact and low cost photometer, which consists of a 505nm LED and a 7cm long MWC, was used for ultra-sensitive detection of oil concentration in O/W emulsion. Due to the enhancement of optical-path and related light-droplet interaction in MWC, detecting diesel of a concentration as low as 2.14 × 10−9g/ml was realized. For the diesel-emulsion detection at visible 505nm, a 125-fold improvement on detection limit was realized compared with that of commercial spectrophotometer. Moreover, in comparison with the conventional ultraviolet detection of diesel-solution at 223nm by using commercial spectrophotometer, a nearly ten-fold improvement on detection limit was realized with MWC-based visible-light detection.
Acknowledgments
This research was supported by grants from the New Century Excellent Talents in the University of China (NCET-05-0111), International S&T Cooperation Program of China (2015DFR10970), the Fundamental Research Funds for Central Universities of China (No. 1302-852005 and 1302-851003), the National Natural Science Foundation of China (No. 61131004 and 61376050). The authors would like to thank Leiming Deng and Zhiwen Wang (School of Mechanical Engineering, Dalian University of Technology) for the spectrophotometry measurement.
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