Portable and Disposable Paper-Based Fluorescent Sensor for In Situ

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A portable and disposable paper-based fluorescent sensor for in situ gaseous hydrogen sulfide determination in near real-time João Flávio da Silveira Petruci, and Arnaldo Alves Cardoso Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.6b03325 • Publication Date (Web): 03 Nov 2016 Downloaded from http://pubs.acs.org on November 4, 2016

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Analytical Chemistry

A portable and disposable paper-based fluorescent sensor for in situ gaseous hydrogen sulfide determination in near real-time

João Flavio da Silveira Petruci* and Arnaldo Alves Cardoso

São Paulo State University (UNESP), Department of Analytical Chemistry, CEP 14800-970, Araraquara, SP, Brazil

e-mail: [email protected] Fax: +551633222308

ACS Paragon Plus Environment


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ABSTRACT Hydrogen sulfide is found in many environments including sewage systems, petroleum extraction platforms, kraft paper mills, and exhaled breath, but its determination at ppb levels remains a challenge within the analytical chemistry field. Off-line methods for analysis of gaseous reduced sulfur compounds can suffer from a variety of biases associated with high reactivity, sorptive losses, and atmospheric oxidative reactions. Here, we present a portable, on-line, and disposable gas sensor platform for the in situ determination of gaseous hydrogen sulfide, employing a 470 nm light emitting diode (LED) and a micro-fiber optic USB spectrometer. A sensing layer was created by impregnating 2.5 µL (0.285 nmol) of fluorescein mercury acetate (FMA) onto the surface of a micro-paper analytical device with dimensions of 5x5 mm, which was then positioned in the optical detection system. The quantitative determination of H2S was based on the quenching of fluorescence intensity after direct selective reaction between the gas and FMA. This approach enabled linear calibration within the range 17–67 ppb of H2S, with a limit of detection of 3 ppb. The response time of the sensor was within 60 s, and the repeatability was 6.5% (RSD). The sensor was employed to monitor H2S released from a mini-scale wastewater treatment tank in a research laboratory. The appropriate integration of optoelectronic and mechanical devices, including LED, photodiode, pumps, and electronic boards, can be used to produce simple, fully automated portable sensors for the in situ determination of H2S in a variety of environments.


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INTRODUCTION Hydrogen sulfide is a flammable gas that is heavier than air and has a characteristic rotten egg odor1. When its concentration in the air is above the olfactory perception threshold (300 ppb), it may present a risk to human health, causing disturbances such as nausea, headache, lung irritation, and even death2. In addition to its toxicity, hydrogen sulfide is corrosive, attacking metallic surfaces, concrete, and electronic components3,4. Estimates suggest5 that 90% of the hydrogen sulfide emitted into the air is derived from natural sources including geothermal activity (hot springs and volcanoes) and emissions from diffuse sources (for example, vegetation and coastal and wetland ecosystems). Natural diffuse emissions usually result in tropospheric concentrations on the order of ppt5,6. Anthropogenic emissions of H2S are related to the extraction and refining of oil and natural gas, sewage treatment plants, manure-handling plants, and tanneries7,8. Most of these processes produce hydrogen sulfide as an industrial byproduct and can cause episodes of high concentrations of the gas (from hundreds of ppb to hundreds of ppm), which can affect public health. Hydrogen sulfide poisoning is the second most common cause of death in workplaces, after carbon monoxide poisoning2. Humans can normally detect the odor of H2S gas at concentrations as low as 10 ppb, but the exposure of workers to hydrogen sulfide can be dangerous when the ability to smell the gas is lost, due to olfactory fatigue, even though it is still present. Loss of this primary warning signal for H2S may be a cause of many fatal gas inhalation episodes. Recommended exposures must not exceed 20 ppm



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(ceiling) and not more than 50 ppm (peak), for a single time period up to 10 minutes9. For these reasons, the in situ determination of hydrogen sulfide in near real-time is essential to ensure workplace safety as well as to assess progress in the management of this pollutant in the wider environment10. A variety of analytical methods for H2S sensing have been described in the literature11–13, and several sensors are commercially available for the occupational health market14. Portable sensors are needed for a variety of different applications. Sensors based on semiconducting metal oxides and electrochemical sensing principles have been used commercially to detect H2S, offering rapid response, low cost, and ease of preparation15,16. Although these devices offer real-time and in situ capability, they have high detection limits on the order of tens of ppm. Moreover, the application of these sensors in real-world sampling scenarios can be questionable, due to a lack of field validation12. Chemical methods with protocols that involve off-line steps (involving field sampling, storage, and laboratory determination) are typically employed for concentrations at the ppb level17–19. Among such procedures, gas chromatography is the gold-standard technique for sensitive H2S detection20. However, drawbacks of these methods include the lack of realtime responses, difficulty of miniaturization and field usage, bulkiness, high cost of instrumentation, employment of dedicated detectors and tube materials, use of large quantities of toxic reagents, and the need for automation when used in harsh environments. An additional problem is that the physicochemical properties of hydrogen sulfide result in its strong adsorption on surfaces, as well as


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oxidation induced by light or in alkaline solutions, which can lead to analytical errors21. In the H2S sampling step, the presence of atmospheric oxidants such as ozone and NO2 may result in significant losses of analyte. Potential biases can also occur in subsequent steps of sample preservation, preparation, and the final measurement, which affect the accuracy of the analysis. Sensors based on optical phenomena (such as absorbance or fluorescence) are ideal for the construction of small, dedicated, devices that are inexpensive, simple to use, highly sensitive, and consume minimal amounts of energy22,23. The emergence of micro-fiber optic spectrometers and light emitting diodes (LEDs) has led to the use of robust, portable, and dedicated

sensors

in

many

industrial,

clinical

(point-of-care),

and

environmental applications23. Fluorescence spectroscopy has often been employed as an analytical technique, due to the sensitivity of the optical signal. In the case of H2S, several luminescent metal-based complexes have been used as sulfide recognition and determination probes. Mercury-complexing agents such as alkaline fluorescein mercuric acetate (FMA) have been widely employed for sulfhydryl group determination24–26. Zinc27, ruthenium28, palladium29, silver30, and copper31 have also been used as luminescent metalbased probes. The classical fluorimetric determination of H2S in the atmosphere, described by Axelrod32, involves collection of the gas in a hydroxide solution (Cd(OH)2 or Zn(OH)2), followed by the measurement of FMA fluorescence at 525 nm, which is quenched after the sulfide reaction. This method generates large quantities of toxic waste and also requires the use of an off-line protocol. There are few studies concerning gaseous H2S determination based on direct



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reaction with FMA or its derivatives. Toda et al.33 described a micro-gas analysis system for on-line, continuous, in situ, and sensitive measurement of H2S, using a gas-permeable membrane on a shallow channel to enable efficient accumulation of the analyte in the FMA solution. In another configuration, a drop of alkaline FMA solution was excited from its interior using an optical fiber and was employed as a renewable sensor for the measurement of low concentrations of atmospheric hydrogen sulfide34. Jayaraman and collaborators25 bubbled mouth air directly into a solution of FMA/NaOH for malodor detection. Table 1 shows some of the methodologies that have employed FMA as a reagent for sulfide determination. Reagents immobilized on solid supports, with simultaneous collection of the analyte and alteration of the reagent optical signal intensity, have been employed for quantitative analysis of liquid and gaseous species35–38. For this purpose, microscale paper-based analytical devices (µPADs) are an alternative to polymeric supports and have the advantages of being disposable, low cost, highly portable, small in size, and easy to handle39,40. In addition, the use of µPADs enables significant reductions in reagent consumption and waste generation, because only µL volumes of reagents are required. In the case of gas analysis, the high porosity of filter paper facilitates gaseous diffusion and reagent-analyte contact40. Although the number of studies describing the use of paper-based chemosensors has dramatically increased, there are few works that have employed µPADs for the direct determination of gaseous analytes. Table 2 lists some of the analytical procedures that have employed paper-based materials for H2S determination.


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This paper describes the development of a portable sensor for in situ gaseous hydrogen sulfide determination. The sensor response was 60 s, enabling its use in applications requiring a rapid analysis and the ability to detect short-term changes in the background concentration of H2S. Quantification was based on quenching of the fluorescence intensity of a microvolume of FMA impregnated on the surface of a paper analytical device (~25 mm2). Excitation using a single LED resulted in emission of light that was detected using an optical fiber and a miniature spectrometer.

EXPERIMENTAL Materials and solvents Whatman 41 cellulose filter paper (Whatman, Kent, England) was used as the solid support. Solutions of fluorescein mercury acetate (Sigma-Aldrich, Germany), sodium hydroxide (Qhemis, São Paulo, Brazil), and ethylene glycol (Synth, São Paulo, Brazil) were prepared using Milli-Q deionized water (Millipore, USA). A longpass (LP) colored glass filter (515 nm), an LED (LED470E), and all the optomechanical components were acquired from Thorlabs (Dachau, Germany). A 600 µm internal core fiber-optic cable was purchased from Ocean Optics (USA).

Generation of gaseous standards Gaseous standards of hydrogen sulfide used for calibration purposes were obtained by the permeation method. An H2S permeation tube (PT) device (VICI Metronics, Santa Clara, USA) was positioned inside a



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permeation chamber (PC) with temperature maintained at 30 ± 0.1 ºC. The PT was certified to release H2S at a rate of 45.83 ng min-1 at 30 ºC. Purified air was obtained by passage through two sequential columns (20 x 400 mm) containing silica gel and activated carbon. Gas flow controllers (FC) were used to deliver different concentrations of hydrogen sulfide to the sampling stream (S). The gas flow through the permeation chamber was fixed at 500 mL min-1. The complete standard gas generator is illustrated schematically in Figure S1 (Supplementary Material).

Reagent solution and preparation of sampling filters A fluorescein mercury acetate stock solution was prepared at a concentration of 10-4 mol L-1 in 0.1 mol L-1 sodium hydroxide. This solution was stored in a dark bottle to avoid exposure to light. The µPADs were prepared by cutting pieces of filter paper with dimensions of 5 x 5 mm. Subsequently, 2.5 µL of FMA solution (0.285 nmol) and 2.5 µL of ethylene glycol were added to the paper before exposure to H2S.

Instrumental system The optical device was assembled using an LED (470 nm) positioned perpendicularly and a miniature spectrometer (HR2000+ES, Ocean Optics, USA) coupled to a fiber optic cable positioned below the sample holder. A longpass colored optical filter was positioned between the paper and the fiberoptic cable entrance. A T-Cube LED driver (Thorlabs, Dachau, Germany) was used to control the LED power. The device is shown schematically in Figure 1.


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The fluorescence spectra and relative fluorescence intensities were acquired and recorded using SpectraSuite software (Ocean Optics, USA), with integration time of 800 ms and bandpass slit of 10 nm.

Analysis protocol Prior to calibration or sample analysis, the µPAD containing FMA and ethylene glycol was prepared and positioned in the optical device, which was then enclosed in a black cardboard box. The paper surface was exposed to an air sample by switching the valves, using a constant gas flow of 500 mL min-1. For real-world samples, a pump (MOA-V112-AE, Gast Manufacturing, USA) was connected to a homemade glass air sampler. This device enabled the air sample to be obtained without passing through the pump. The sampling flow was calibrated using appropriate valves. The fluorescence intensity at an emission wavelength of 525 nm was recorded within 3 min, and the LED was set to switch on for 5 s every 1 min.

RESULTS AND DISCUSSION Evaluation of signal stability Fluorescein mercury acetate has a very strong emission band centered at 525 nm when excited in the blue region (450–490 nm) in sodium hydroxide solution. Firstly, the emission of FMA impregnated on the paper surface was evaluated using an excitation wavelength of 470 nm. The emission spectrum is shown in Figure 2. Continuous exposure of organic dyes to high levels of irradiation can cause degradation of the molecules and therefore diminish their optical properties. It has been reported previously that FMA is rapidly



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degraded by light24. The H2S determination is based on quenching of the fluorescence intensity of FMA, so it is essential to be able to distinguish the signal resulting from reaction with the analyte from that produced by photoquenching, in order to avoid false positive signals. The geometry of the optical arrangement and the duration of exposure of the FMA solution to irradiation are the most important parameters to optimize in order to achieve the best sensitivity without dye degradation. Firstly, the LED was positioned directly above the paper surface impregnated with FMA, with the fiber optic cable positioned directly below the paper surface, and the signal was recorded continuously for 200 s. It was found that optical power greater than 100 mW caused strong degradation of the dye, so this geometry was not used in subsequent experiments. The LED was then positioned perpendicular to the paper surface, allowing exposure to a portion of the radiation, and the LP filter and fiber optic cable were again positioned below the paper (Figure 1). Figure 3 shows the intensity of the signal monitored during 350 s using this geometry. Slight degradation of the dye still occurred with optical power of 170 mW. In order to minimize this effect, the LED was set to emit intermittent 5 s light pulses, which resulted in no change in the signal during 350 s at the maximum optical power (figure 4).. This geometry was therefore used in all the subsequent experiments.

Effect of sampling flow on the sensor response The efficiency of trapping molecules that enter the sensing layer is an important parameter to be established in order to optimize the sampling conditions of the analytical method40. The sampling flow was passed through


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the µPAD containing FMA and ethylene glycol during 3 min. The H2S concentration was maintained at 67 ppb and the gas flow rate ranged from 100 to 1000 mL min-1. The sensor response (α) was defined as the difference between the fluorescence intensity of the blank (I0) and the fluorescence after H2S exposure (I). Using an emission wavelength of 525 nm, the response increased with sampling rate up to a maximum at 500 mL min-1, after which higher sampling flows resulted in lower sensor response (Figure 5). At high flow rates, shorter contact time of H2S molecules with the FMA reagent was likely to affect the efficiency of the reaction. The best sampling efficiency was obtained at a flow rate of 500 mL min-1, which was therefore used in all subsequent experiments.

Effect of ethylene glycol on the sensor response In our earlier work, impregnation using ethylene glycol together with reagents was found to improve gas sampling procedures40–42. This compound acts as a humectant, facilitating solid/gas phase interactions by increasing the number of water molecules on the solid surface and enabling higher inputs of gas molecules to the solid layer. A four-fold increase of the response was obtained when the cellulose surface was impregnated with 2.5 µL of ethylene glycol.

Evaluation of sensor performance The sensor was calibrated by determining the response as a function of H2S concentration. The analytical response (α) was the difference between the fluorescence intensities of the blank (I0) and the sample (I), at an emission



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wavelength of 525 nm. The sampling flow rate was 500 mL min-1 and the sampling time was 1 min. A linear relation was obtained for hydrogen sulfide concentrations between 17 and 67 ppb, and for each concentration, an average of three independent replicates was calculated. The equation obtained was:

α = 27.2 [H2S] + 955.2

(r2 = 0.987)

(Equation 1)

Figure 6 shows the behavior of the analytical signal for H2S during 3 min of sampling, recording the sensor response every 60 s. Within the range of concentrations evaluated, the signals were proportional to the sampling time. Therefore, the analytical parameters should consider the sampling conditions. For example, the limit of detection could be improved by increasing the sampling time. Under the experimental conditions employed, with a 1 min sampling time, the relative standard deviation (RSD) of the measurements (n=7) was less than 6.5%. Based on three times the standard deviation of the blank signal, the calculated limit of detection was 3 ppb. The sensor performance parameters are summarized in Table 3.

Field measurements The complete sensing platform (200 x 200 mm) was positioned in a laboratory with a mini-scale wastewater treatment tank at the Institute of Chemistry of São Paulo State University. Wastewater containing different concentrations of sulfate was biotreated by bacteria, reducing SO42- to H2S. The air inside the laboratory was sampled into a flask containing an


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absorption solution to collect the H2S. The concentration of H2S obtained was used as a reference. The sensing platform was then positioned in several locations, including at the flask output, above the flask, at the opposite side of the laboratory, and outside the laboratory. Gas samples were directed to the platform using a homemade glass air sampler connected to a pump. The device provided aspiration of the desired air sample based on the Venturi effect, enabling analysis of the air without passing through the pump. This procedure was essential to avoid losses of H2S due to absorption on the metal and rubber components of the pump. The suction flow could be described by the formula: FSUCTION = (FWASTE + FSAMPLING) – FPUMP (Equation 2). All the gas flows were measured using a primary standard air flow calibrator (Sensidyne, St. Petersburg, USA). The sampling gas flow was set at 500 mL min-1. A schematic illustration of the sampler device and the flows is provided in Figure 7. Photographs of the complete assembled platform used for H2S monitoring in the laboratory are provided in Figures S2 – S4 (Supplementary Material). The results (Figure 8) revealed very high concentrations of H2S in the flask output air and in the region above the collection flask. The FMA fluorescence intensity was completely quenched to background within 1 min of sampling, indicating that the hydrogen sulfide concentration exceeded the linear range employed in this study. At the opposite side of the laboratory, H2S was detected at a concentration of 25 ppb, indicating that workers were constantly exposed to high levels of H2S, even at around 10 meters from the H2S collection flask. Most of the individuals present reported loss of olfactory perception of hydrogen sulfide within several hours, reaffirming the need to



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monitor H2S using near real-time measurements. Table 4 shows the obtained results of H2S monitoring in the laboratory. In figure S-5 is presented a scheme containing the analyses spots distributed in the lab.

Conclusion A portable platform was developed for the in situ determination of hydrogen sulfide. The system consisted of an inexpensive LED emitting at 470 nm and a micro-fiber optic USB spectrometer with total dimensions of 200 x 200 mm. The detection of hydrogen sulfide was based on fluorescence quenching due to the selective and sensitive reaction between H2S and fluorescein mercuric acetate impregnated onto a paper analytical device with small dimensions (5 x 5 mm). A microvolume of FMA (2.5 µL; 0.285 nmol) was employed for each analysis, hence minimizing the amount of toxic waste generated in the sulfide analysis. The use of paper as the solid support offered the advantages of disposability, low cost, easy handling, and minimal environmental impacts. The sensor was able to detect 3 ppb of H2S, with a response time of 60 s. The complete portable platform was employed to monitor hydrogen sulfide in several locations of a research laboratory in which a mini-scale wastewater treatment tank was installed. The proposed method offers superior analytical performance among others described in the literature that employs either FMA or paper-based substrates, with the advantage of, simultaneously, perform the gas sampling and the analytical signal acquisition in near real-time on an inexpensive, disposable and dimension reduced material. Although a power source is required, the technique is suitable for in situ use in real-world measurements of H2S. With


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further development of the sampling unit, the use of portable and calibrated pumps, together with upgrading of the optoeletronic parts of the platform, should enable production of a fully automated device for in situ determination of H2S in various situations.

Acknowledgements The authors are grateful for the financial support provided by the São Paulo State Research Foundation (FAPESP, grants 2013/22995-4 and 2015/23265-5). We would like to thank Prof. Dr. Maria Angélica Martins for assistance in design of the glass air sampler.

Supporting Information Additional information as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org. Scheme of the standard gas generator and the spots where H2S determinations were carried out can be seen as well as photos of the proposed platform.

Conflict of interest

The authors declare no competing financial interest.

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Tables Table 1. FMA-based methods for gaseous H2S sensing. FMA-based

Sensor device

Response

FMA quantity

time

per analysis

< 8 min

4 µmol

37

< 3 min

1 mL / 5 µM

43

< 4 min

1 mL / 1 µM

33

Bubbled in NaOH

Up to 60

2 mL / 1 µM

32

solution

min

Bubbled in

< 5 min

5 mL / 1 µM

25

1 min

2.5 µL / 5 µM

This

probe TOTA - FMA

Ethyl cellulose based

Ref

membrane FMA

PTFE scrubber and flowing FMA solution

FMA

Microchannel scrubber

FMA FMA

FMA/NaOH solution FMA

Filter paper (µPAD)

work



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Table 2. Paper-based sensors for H2S determination. Detection

Response

LOD

Sensor

Ref

Method

time

Electrochemical

20 min

15 ppm

Polyaniline

44

Electrochemical

20 min

10 ppm

Copper acetate

45

Optical

15 min

2 ppb

Palladium-based

40

Visual

30 min

30 ppb

Bismuth-based

46

Optical

10 s

50 ppb

Lead acetate

47

Optical

2.5 min

-

Methyl green

48

Optical

60 s

3 ppb

FMA

This work


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Table 3. Analytical parameters and sensor performance. Parameter

Value

Linear range

17–67 ppb

Correlation coefficient

0.99

Limit of detection (3*SD of blank)

3 ppb

Calibration equation

α = 27.2 H2S + 955.2

Response time

60 s

Precision (RSD, n=7)

6.5%

Paper dimensions

5 x 5 mm

FMA quantity

0.285 nmol



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Table 4. Results of H2S monitoring (ppb) in a wastewater treatment laboratory. Spots are showed in figure S-3 (Supplementary Material).

[H2S]

Spot 1

Spot 2

Spot 3

Spot 4

Spot 5

Spot 6

> range

> range

25 ± 3

23 ± 2

17 ± 3

N.D.


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Figure 1. Schematic illustration of the sensing platform. The LED is positioned at 90º to the paper device.



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Figure 2. Emission spectrum of fluorescein mercuric acetate impregnated onto a paper surface, with excitation wavelength of 470 nm.


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Figure 3.. Fluorescence intensity monitored at 525 nm under continuous irradiation at 470 nm using different powers, during 300 s.



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Figure 4. Fluorescence intensity monitored at 525 nm under pulsed irradiation from the excitation source, using different powers.


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Figure 5. Sensor response for different gas flows containing 67 ppb H2S.



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Figure 6. Sensor response (α) for different H2S concentrations during 180 s.


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Figure 7. Schematic illustration of the glass air sampler based on the venturi effect.



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Figure 8. Sensor response for air samples obtained at different locations in a wastewater treatment research laboratory.


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“For TOC only”


Portable and Disposable Paper-Based Fluorescent Sensor for In Situ

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