Sensors and Actuators B 134 (2008) 234–237
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical
journal homepage: www.elsevier.com/locate/snb
Low-cost colorimetric sensor for the quantitative detection
of gaseous hydrogen sulfide
Avijit Sen, Jeffrey D. Albarella, James R. Carey, Patrick Kim, William B. McNamara III ∗
ChemSensing Inc., Champaign, IL 61820, USA
a r t i c l e
i n f o
Article history:
Received 27 November 2007
Received in revised form 21 April 2008
Accepted 23 April 2008
Available online 6 May 2008
Keywords:
Colorimetric sensor
Hydrogen sulfide sensor
Sensor array
Gas detection
a b s t r a c t
A simple, rapid and low-cost method for differentiating a wide range of concentrations of gaseous hydrogen sulfide (H2 S) is described. A large dynamic range (50 ppb to 50 ppm) was successfully classified at
ambient temperature using a colorimetric sensor array comprised of chemoresponsive dyes coupled to a
commercially available 8-bit flatbed scanner. Principle component analysis of the composite magnitude
and kinetics of color change for the array of dye spots over the entire dynamic range resulted in 100%
classification. This was achieved by including chemoresponsive dyes which reacted specifically with H2 S
at different detection thresholds.
© 2008 Elsevier B.V. All rights reserved.
1. Introduction
Hydrogen sulfide (H2 S), a highly toxic, corrosive and flammable
gas, is produced in sewage, coal mines, oil and natural gas industries, and is utilized in many industrial chemical applications. It has
an occupational exposure limit of 10 parts per million (ppm) for 8 h
exposure, and its odor detection threshold ranges from as low as
0.02 ppm to a maximum of roughly 5 ppm [1]. Recently, there has
been much interest in the development of real time [2], portable [3],
and optical [4] hydrogen sulfide detectors operable under ambient
conditions [5,6] and capable of detection at the parts per billion
(ppb) level [7]. While there are many commercially available H2 S
sensors, none are colorimetric [8], inexpensive, and disposable sensors capable of detecting a wide range of concentrations of H2 S at
ambient temperature. We describe herein the use of an inexpensive
chemical sensor array (CSA) for the detection and classification of
a wide range of concentrations (50 ppb to 50 ppm) of H2 S at room
temperature.
The CSA discussed here consists of an array of chemoresponsive
dyes [9] that undergo a color change upon interacting with gases
and vapors. For the purpose of the work described in this report,
the gas is H2 S. The CSA used in this report is shown in Fig. 1.
The dye formulations included in CSAs were chosen based on
three primary criteria. First, the formulation must react strongly
with one or more analytes. The interaction must not be simple
∗ Corresponding author. Tel.: +1 217 328 3270; fax: +1 217 328 3207.
E-mail address: [email protected] (W.B. McNamara III).
0925-4005/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.snb.2008.04.046
physical adsorption, but rather must involve stronger chemical
interactions. Second, the reaction site must be strongly coupled to
an intense chromophore such that reaction with an analyte leads
to a measurable color change. Third, the formulation must not be
so reactive that it precludes reproducible manufacturing and longterm stability during storage. A fourth criterion is imposed upon
those dyes that meet the first three. Given that the goal of this work
was the development of an H2 S sensor that would function over a
broad concentration range, the CSA must possess chemoresponsive
dyes which react specifically with H2 S and have the required range
of chemical sensitivity.
2. Principle of a CSA experiment
A chemical or gas sensing experiment is simple in concept;
the dye formulation spots change color during exposure to various gases. The composite magnitude and kinetics of the response
for the array of dye spots identify the gas and its concentration.
These properties are measured by collecting an image of the array
prior to exposure to an analyte and then at regular time points after
exposure commences.
Proprietary image analysis software locates each dye spot in the
CSA, samples approximately 300 pixels in the center of each dye
spot, and determines the average red, green, and blue (RGB) values for each dye spot. Each of the three colors for a given dye spot
constitutes a single “color channel.” For example, the red value for
dye spot 16 is an individual color channel denoted R16 . Specific dye
spot numbers are determined using the system described in Fig. 1.
The color change for each dye spot at a particular time point can
A. Sen et al. / Sensors and Actuators B 134 (2008) 234–237
235
The resulting data can be expressed as a digital array of color
change values; a vector of 3N dimensions where N equals the number of dye spots present in the CSA (i.e. 3 × 36 = 108 elements per
exposure time point for the experiments described here). Further,
as images were collected at multiple exposure time points, each
trial yields an M × 108 digital array where M is the number of exposure time points.
3. Materials and experimental setup
3.1. CSAs
Fig. 1. Disposable CSA, ChemSensing Inc., Champaign, IL. Part No. CSI.082. The dye
spot numbering system progresses from left to right, top to bottom, i.e. the top row
consists of spots 1–6, the second row spots 7–12, etc (image has been enlarged for
clarity).
be determined by taking the difference in RGB values between a
post-exposure image for that time point and those of the baseline
(pre-exposure) image. For example, the color change in dye spot 16
following 10 min exposure is given by Eq. (1).
RGB16,10 = (R16,10 − R16,0 ), (G16,10 − G16,0 ), (B16,10 − B16,0 )
(1)
All CSAs used in the experiments described herein were manufactured by ChemSensing Inc., Champaign, IL, part number CSI.082.
Please see www.chemsensing.com for more information. All arrays
used consisted of 36 unique dye formulation spots, which were
non-contact printed onto a porous reflective substrate in a 6 × 6
matrix. Formulations typically consist of dye, solvent, and additives which function to maintain the dye in its desired chemical
state after it has been dispensed during the CSA manufacturing process. The formulations used in the CSA include metalloporphyrins,
pH indicators, and solvatochromic dyes among others [10]. Printed
CSAs are packaged in an optically clear cartridge, as shown in Fig. 1,
and hermetically sealed in a pouch to prevent contamination during storage. The CSAs used for the experiments described here
included dye formulations with differing thresholds for H2 S detection, intended to enable differentiation of H2 S concentration over
a wide range.
3.2. Analyte/gas mixture generation
All H2 S concentrations described in this report were generated
using a system of two mass flow controllers (Model 1479, MKS
Instruments, Wilmington, MA). The H2 S delivery system and array
imaging platform are shown schematically in Fig. 2.
Gas cylinders of certified dilute H2 S (in N2 ) were obtained from
Matheson Tri-Gas. These cylinders served as the source for subsequent dilution with humidified N2 to reach a desired analyte
Fig. 2. Gas mixing system used to deliver specified H2 S concentrations to CSAs.
236
A. Sen et al. / Sensors and Actuators B 134 (2008) 234–237
Fig. 3. Representative response for Formulation A, showing magnitude of color
change over the course of 30 min of exposure to various concentrations of H2 S.
concentration. Humidified N2 was generated by bubbling dry N2
through water in a closed container. For all generated concentrations, a flow rate of 200 standard cubic centimeters per min was
delivered to the sensor during exposure. All trials were carried out
at ambient temperature (21–26 ◦ C).
3.3. Sensing experiment
Sensing experiments consist of collecting images of the CSA both
prior to exposure to H2 S and at specified time points after exposure
commences. Images were collected using an off-the-shelf flatbed
scanner (Epson Perfection 3490 Photo). The scanner was driven
by proprietary software designed to control imaging parameters
in order to reduce any effect of variation in imaging conditions
on experimental data. To begin an experiment, the cartridge is
removed from the hermetically sealed pouch and inserted into a
fixture containing two needles. These needles puncture the gasket
at the base of the cartridge as the cartridge is inserted and serve as
the inlet and outlet for analyte delivery to the CSA. In addition to
interfacing the CSA to the analyte delivery system, the fixture holds
the CSA in an identical fixed position on the scanner bed for each
trial.
Replicate CSA response data were collected for several concentrations, expressed as concentration (number of replicates): 50 ppb
(3), 100 ppb (4), 250 ppb (4), 500 ppb (3), 1 ppm (3), 2 ppm (4),
5 ppm (3), 10 ppm (6), 25 ppm (4), 50 ppm (4), and humidified N2
(7). CSAs were exposed for a total of 30 min, with images collected
at 0, 1, 2, 3, 5, 7, 10, 15, 20, 25, and 30 min time points. The resulting images were analyzed as described in Section 2 to yield, for
each trial, a unique numerical array of color change values that is
(exposure time points) by (color channels) in dimension: a 10 × 108
element array.
rate of color change. Formulation A has high classification resolution for low concentrations, but possesses little classification power
for concentrations in excess of 5 ppm (not shown). In contrast, Formulation B, shown in Fig. 4, has a sensitivity threshold suited for
differentiation of higher concentrations of H2 S, while possessing
minimal reactivity to sub-ppm concentrations (not shown). Little
difference in response is demonstrated for Formulation B between
1 and 2 ppm. However, 5, 10, 25 and 50 ppm can be readily differentiated within 10 min exposure. As discussed below, this broad range
of sensitivity, coupled with differing response kinetics, allows a single disposable CSA to differentiate H2 S concentrations over at least
three orders of magnitude.
In other trials (not shown here), neither Formulation A nor
B showed response above noise when exposed to immediately dangerous to life and health (IDLH) concentrations of:
acrolein (2 ppm), acrylonitrile (85 ppm), ammonia (300 ppm),
arsine (3 ppm), chlorine (10 ppm), cyanogen chloride (20 ppm),
ethylene oxide (800 ppm), formaldehyde (20 ppm), hydrogen chloride (50 ppm), hydrogen cyanide (50 ppm), phosgene (2 ppm), and
sulfur dioxide (100 ppm).
4.2. Incorporation of response kinetics in classification of H2 S
concentration
As illustrated in Figs. 3 and 4, differences in dye formulation
response rate as a function of H2 S concentration provide important
classification information. To incorporate response kinetics into the
classification model, “time-stacked” vectors were constructed for
each replicate trial. The time-stacked vector, ␯, from any one trial
is given by Eq. (2), using images collected at 0, 1, 2, 3, 5, 7, 10, 15,
20, 25, and 30 min. The 0 min image was used as the baseline for
calculating differences in RGB values:
= R1,1 min , G1,1 min , B1,1 min , R2,1 min , G2,1 min ,
B2,1 min , . . . , R36,1 min , G36,1 min , B36,1 min , R1,2 min ,
G1,2 min , B1,2 min , . . . , R36,30 min , G36,30 min , B36,30 min
(2)
Given the multivariate nature of CSA response data, the timestacked vectors were processed using principal component analysis
(PCA) [11,12]. PCA is often used to eliminate redundant variables
and reduce the dimensionality of a data set, allowing for facile data
visualization. PCA operates on a set of data to create a new coordinate system such that the first coordinate, termed the first principal
component (PC), captures the maximum amount of variance possible in the original data set. The second PC is that coordinate,
orthogonal to the first PC, that captures the maximum possible
amount of the residual variance, and so on. A three-dimensional
4. Results and discussion
4.1. CSA response to various H2 S concentrations
Representative responses for two dye formulations to several
different concentrations of H2 S are shown in Figs. 3 and 4. The
figures show the magnitude of color change over the course of
30 min of exposure to H2 S. As discussed above, the dye formulations
demonstrate a wide range of H2 S sensitivity. Formulation A, shown
in Fig. 3, possesses a high level of differentiation for low concentrations of H2 S. The lowest concentrations (50, 100, 250, 500 ppb)
show marked differences in magnitude of color change after 30 min
of exposure. Also evident are differing rates of response; while 2
and 5 ppm produce a rapid initial response and reach saturation
at 15 min, lower concentrations show a more gradual and constant
Fig. 4. Representative response for Formulation B, showing magnitude of color
change over the course of 30 min of exposure to various concentrations of H2 S.
A. Sen et al. / Sensors and Actuators B 134 (2008) 234–237
237
within 2 min of exposure, the CSA system may be particularly well
suited for environmental monitoring applications in which a rapid
warning of unsafe levels of H2 S is needed.
5. Conclusion
Fig. 5. PCA scores plot for several concentrations of H2 S resulting from analysis of
time-stacked vectors.
A colorimetric H2 S sensor capable of detecting and classifying H2 S concentration over three orders of magnitude at ambient
operating temperature has been developed. Ten different concentrations of H2 S were correctly classified using simple methods of
statistical analysis. By incorporating dye formulations possessing
a wide range of sensitivity to H2 S, dangerously high concentrations can be rapidly identified and sub-ppm concentrations can
be accurately classified using the same sensor array. The low-cost
and disposable nature of CSA technology lends to its use for environmental monitoring. The development of a hand-held device is
currently underway. Further development is focused on decreasing the time needed to classify sub-ppm concentrations of H2 S,
both through altering formulation chemistry and optimizing CSA
geometry.
References
PCA score plot (Fig. 5) shows clear clustering of 45 trials carried out
at 11 different H2 S concentrations. It should be noted that even at
concentrations as low as 50 ppb, our CSA technology generates a
reproducible response which is differentiable from humid N2 .
4.3. Environmental monitoring
It is understood that 30 min might be an unacceptable length of
time for determining high concentrations of H2 S. OSHA permissible
exposure limits (PEL) for H2 S, for example, range between 10 and
20 ppm. Legal limits for workplace concentrations may measure
between 20 and 50 ppm, for a single time period of up to 10 min,
if no other measurable exposure occurs throughout the same 8 h
work shift. It is therefore useful to be able to detect high concentrations of H2 S in minutes rather than tens of minutes, and
the development of a hand-held, CSA-based portable device for
that purpose is currently underway. Fig. 6 illustrates the ability of
the CSA to rapidly detect and identify relatively high concentrations of H2 S. This PCA plot was created using a subset of the data
described above; only time-stacked vectors for 1 and 2 min data
from exposure to PEL relevant concentrations were considered. As
concentrations in the PEL range can be detected and differentiated
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Biographies
Avijit Sen received his Ph.D. from the Indian Institute of Science, Bangalore, India,
in 1997 and his B.Sc. and M.Sc. degrees, from Jadavpur University, Kolkata, India, in
1989 and 1991, respectively. He is a founding scientist of ChemSensing Inc.
Jeffrey Albarella received his B.S. degree from Purdue University, West Lafayette,
IN, in 2000. He has worked with ChemSensing Inc. since 2004.
James R. Carey received his Ph.D. from the University of Illinois, Urbana-Champaign,
IL, in 2006, his M.S. from the University of Maryland, College Park, MD, in 1997 and
his B.S. from Rutgers University, Camden, NJ, in 1995. He has been a Senior Research
Scientist at ChemSensing Inc. since 2006.
Patrick Kim received his B.S. from the University of Illinois, Urbana-Champaign, IL,
in 2004. He has worked with ChemSensing Inc. since 2004.
William McNamara received Ph.D. from the University of Illinois, UrbanaChampaign, IL, in 2000 and his B.S. from the University of California, Santa Barbara,
CA, in 1992. He is a founding scientist and the current Chief Technical Officer of
ChemSensing Inc.
Fig. 6. PCA scores plot for PEL relevant concentrations of H2 S resulting from analysis
of time-stacked vectors for 1 and 2 min data.