Portable device for the detection of colorimetric assays

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rsos.royalsocietypublishing.org
Research
Cite this article: Luka GS, Nowak E, Kawchuk
J, Hoorfar M, Najjaran H. 2017 Portable device
for the detection of colorimetric assays. R. Soc.
open sci. 4: 171025.
http://dx.doi.org/10.1098/rsos.171025
Received: 31 July 2017
Accepted: 27 September 2017
Subject Category:
Engineering
Subject Areas:
bioengineering/analytical
chemistry/environmental chemistry
Keywords:
colorimetric detection, portable device, point
of care, sensors, nitrite detection, pH detection
Authors for correspondence:
G. S. Luka
e-mail: gswluka@hotmail.com
H. Najjaran
e-mail: homayoun.najjaran@ubc.ca
Portable device for the
detection of colorimetric
assays
G. S. Luka, E. Nowak, J. Kawchuk, M. Hoorfar and
H. Najjaran
School of Engineering, University of British Columbia, 333 University Way, Kelowna,
British Columbia, Canada V1V1V7
GSL, 0000-0002-6567-3607
In this work, a low-cost, portable device is developed to
detect colorimetric assays for in-field and point-of-care (POC)
analysis. The device can rapidly detect both pH values and
nitrite concentrations of five different samples, simultaneously.
After mixing samples with specific reagents, a high-resolution
digital camera collects a picture of the sample, and a singleboard computer processes the image in real time to identify
the hue–saturation–value coordinates of the image. An internal
light source reduces the effect of any ambient light so the
device can accurately determine the corresponding pH values
or nitrite concentrations. The device was purposefully designed
to be low-cost, yet versatile, and the accuracy of the results
have been compared to those from a conventional method.
The results obtained for pH values have a mean standard
deviation of 0.03 and a correlation coefficient R2 of 0.998. The
detection of nitrites is between concentrations of 0.4–1.6 mg
l−1 , with a low detection limit of 0.2 mg l−1 , and has a mean
standard deviation of 0.073 and an R2 value of 0.999. The results
represent great potential of the proposed portable device as an
excellent analytical tool for POC colorimetric analysis and offer
broad accessibility in resource-limited settings.
1. Introduction
Recently, there has been a growing demand to monitor and
control all aspects of our urban environment in real time. This
is brought about by increasing concerns with environmental,
health, security and safety issues. Hence, there is a need to
develop low-cost, versatile and portable diagnostic tools for
early detection of harmful contaminants readily and accurately
in remote areas [1,2]. Many analytical tools have been used
in the field of medical diagnostics, environmental applications,
2017 The Authors. Published by the Royal Society under the terms of the Creative Commons
Attribution License http://creativecommons.org/licenses/by/4.0/, which permits unrestricted
use, provided the original author and source are credited.
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food analysis, and security and defence. These tools rely on polymerase chain reaction (PCR) [3–
5], high-performance liquid chromatography (HPLC) [6], mass spectroscopy [7,8] and enzyme-linked
immunosorbent assay (ELISA) [9,10] for analysis. These standard methods for the detection of specific
targets possess high accuracy and sensitivity. However, they are expensive, bulky, time-consuming,
complicated and need technological improvement, and require trained personnel, which together hinder
their applicability for point-of-care (POC) applications [1,11–13].
Advances in digital imaging technology allow for the implementation of sensitive detection
and processing methods using cost-effective and high-resolution cameras for colorimetric analysis.
It is now possible to exchange results of POC tests in remote areas with subject matter experts
without delay by using smartphones [14]. More recently, cloud computing [14] has provided a
remarkable promise towards bioanalytical needs, mobile environmental, medical diagnostics and
the development of POC testing. Cloud computing enables immediate data transmission and
access to information for food, environmental and medical analysis centres worldwide via wireless
connections [15], especially in resource-limited settings and remote areas, enabling intervention when
required [16].
Colorimetric methods have proved to be fast, adaptable, cost-effective and sensitive for the detection
of a wide range of different analytes [17–20] (in a single test) [21,22] ranging from toxic chemicals
[23–25] to pathogens [26–28], which can be detected using either the naked eye or optical instruments.
However, they are limited to laboratory-controlled settings [29] due to the complexity and fragility of the
instruments used for detection. They also need well-trained personnel to run the instruments, which may
hinder early diagnostics in remote areas [30–32]. As a result, the integration of these colorimetric assays
into portable, versatile, cost-effective, fast and reliable optical digital detectors with onboard processing
abilities will: (i) increase the impact and usage of these sensing methods, (ii) reduce time and cost of
such tests [33], (iii) provide reliable and rapid analysis and (iv) provide the means for POC diagnostic
tools [34,35].
Here, we report the development of a low-cost portable diagnostic device for rapid pH determination
and nitrite concentration detection (as a proof of concept) to detect colorimetric assays for in-field
and POC diagnostics. Nitrite was chosen as one of the analytes to be detected because of the
major worldwide concern about its pollution in water. Nitrite is widely used as a preservative
and additive in food production, bleaches and dyes in the industry, and fertilizer in agriculture.
However, it is classified as a health hazard because high nitrite concentration in water can cause
many diseases such as methaemoglobinaemia [36], miscarriages and central nervous system defects at
birth [37].
The developed device is battery-powered, wirelessly connected and equipped with a low-cost, singleboard computer (Raspberry Pi) for data processing, storage and exchange. A light-emitting diode
(LED) provides backlighting, and an 8-megapixel resolution digital camera is used to image and record
the colour change in five different cuvettes containing the specific reagents required to perform the
colorimetric assay. A rotating carousel sitting on an electromotor is used to position the cuvettes
automatically in front of the camera consecutively (figure 1). To enable fast prototyping, the device
and its components were fabricated using three-dimensional (3D) printing technology. The use of a
stable and reliable external light source and a fixed distance between the sample and the camera in
the designed device has allowed the device to be used in different light conditions. Also, it has decreased
the interference from ambient light and improved measurements reproducibility. These two drawbacks
significantly limit the use of smartphones in sensing technologies and for POC applications [38,39]. Using
a standalone device rather than a smartphone also reduces the potential setbacks caused by new models
of phones in the future and any incompatibilities that may arise [40,41].
The device also incorporates onboard electronics that calculate saturation value of the (hue–
saturation–value, HSV) coordinates of the recorded images. The presence of nitrite or change in the
pH value of the sample causes a detectable colour change in the sample cuvette. The cuvette is then
placed in one of the five slots of a rotating carousel. An image of the assay is taken with the integrated
camera and digitally processed using the single-board computer in real time. Hue and saturation
coordinates of the HSV colour space are used to determine the pH and nitrite concentration in the
sample, respectively. The image-processing algorithm used decreases the effect of ambient light when
the picture is taken and adjusts the cuvette’s position in the image frame. Compared with images
taken by smartphones for the detection of colorimetric assays [42], the use of a fixed camera in the
proposed portable device (as demonstrated in this work) reduces the likelihood of human error, which
can compromise repeatability, sensitivity and reliability of colorimetric assays for personal use and
POC testing.
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cuvette
LED
3
lid
camera
stepper
motor
raspberry Pi
Figure 1. The portable colorimetric device and its major components.
Table 1. Major hardware components and their costs.
part
(i) Raspberry Pi Model B
cost (USD)
$45
(ii) Raspberry Pi Camera Module
$30
.........................................................................................................................................................................................................................
.........................................................................................................................................................................................................................
(iii) 2.8 PiTFT Resistive Touchscreen 1.5
$35
(iv) VeroWhite 3D Printing Material
$150
total
$260
.........................................................................................................................................................................................................................
.........................................................................................................................................................................................................................
.........................................................................................................................................................................................................................
2. Material and methods
2.1. Colorimetric assay preparation
For the preparation of the pH-sensitive colorimetric assay, bromothymol blue (CAS no. 76-59-5),
methyl red (CAS no. 493-52-7), sodium hydroxide (CAS no. 1310-73-2), ethyl alcohol (ethanol) (CAS
no. 64-17-5), sodium phosphate monobasic monohydrate (CAS no. 10049-21-5) and sodium phosphate
dibasic (CAS no. 7558-79-4) were purchased from Sigma-Aldrich (Canada). For the preparation of the
nitrite colorimetric assay, N-1-naphthylethylenediamine dihydrochloride (NED) (CAS no. 1465-25-4),
sulfanilamide (CAS no. 63-74-1) and sodium nitrite (CAS no. 7632-00-0) were purchased from Sigma
(Sigma-Aldrich, Canada).
2.2. Portable device fabrication
This section outlines the fabrication and construction of the electronic and hardware components
of the portable colorimetric device (figure 1). Table 1 outlines the major hardware components and
their costs.
The electronic hardware components used to construct the device are a Raspberry Pi computer (i),
Raspberry Pi camera module (ii) and 2.8 PiTFT touchscreen (iii). These are used to collect image
samples, process the collected images and display the nitrite concentration and pH values to the user. The
Raspberry Pi Model B single-board computer was selected as the embedded controller for the device due
to its low-cost, small size (the size of a credit card) and availability of open source support libraries. Power
leads were soldered to test points 1 and 2 (TP1 and TP2) on the Raspberry Pi board to supply 5 V power
directly to the board, bypassing the USB connector and onboard power regulators. Power is supplied
to the onboard electronics by a 3.7 V 2500 mAh Lithium polymer battery. The battery is connected to a
PowerBoost 500 C charging circuit from Adafruit Industries. This allows the device to be charged using
................................................
display screen
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wheel
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pH
4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0
a 5 V micro-USB connector and boosts the 3.7 V output of the battery to 5.2 V required by the Raspberry
Pi and stepper motor. This option allows the portable device to operate on battery power for several
hours and allows for simple charging using a standard micro-USB phone charger.
The device can be controlled by a user-friendly touchscreen interface to simplify operation. The 2.8
resistive PiTFT touchscreen display is mounted at an angle on the front of the device; a resistive display
was chosen over a capacitive screen to allow the operator to control the device while wearing gloves.
Samples are loaded into rotating cuvettes, and a 5 V geared stepper motor positions the samples in
front of the camera. A ULN2803 8-Channel Darlington Driver IC is used to control the stepper motor
and activate a white super bright LED from the Raspberry Pi’s general purpose input output (GPIO)
pins when capturing images. The LED was placed in the centre of the rotating carousel, allowing it to
backlight the cuvette being imaged.
The stock Raspberry Pi camera lens was modified to reduce the working distance to 15 mm by
unscrewing the lens to its outermost thread. The device was outfitted with a RTL8188eu Wi-Fi chip
enabling connection to a 2.4 GHz wireless network to transfer data for external storage or further analysis
on an external computer. A three-dimensional-printed enclosure was designed to house all components
inside a compact unit.
2.3. pH and nitrite colorimetric assay preparation
The colorimetric assay for the detection of pH was prepared to cover the required pH range from 4 to
8. Two different acid–base indicators: methyl red ranging (from 4 to 6) and bromothymol blue ranging
(from 6 to 8) were used for the detection of pH. The pH range was selected based on the acceptable range
for drinking water in most worldwide regulations [43]. A mixture of the two indicators was prepared to
measure a wider pH scale (from 4 to 8) in one sample (figure 2). To measure the pH value, the mixture of
two pH indicators (double indicators) was used. The first indicator was prepared by dissolving 0.02 g of
methyl red in 60 ml of ethyl alcohol. Then, DI water (with a resistivity of 18.2 MΩ using the Millipore
Synergy water purification system) was added to increase the volume of the solution to 40 ml. For
the second indicator, 0.1 g of bromothymol blue was dissolved in 16.0 ml of NaOH (0.01 M). Then, the
solution was diluted to 250 ml with DI water. For the pH colorimetric assay, 250 µl of bromothymol blue
and 50 µl of methyl red were used. For the nitrite concentration determination, two different solutions
and four different concentrations of sodium nitrate were prepared: (i) 1% sulfanilamide solution was
prepared by dissolving 0.15 g of sulfanilamide in 15 ml of 3 M HCl and (ii) 0.02% N-1-napthylethylene
diamine dihydrochloride solution was prepared by dissolving 2 mg of N-1-napthylethylene diamine
dihydrochloride in 100 ml of DI water.
The colorimetric assay for the deamination of the nitrite concentration was prepared by adding
different nitrite concentrations to the colourless nitrite assay reagents, which resulted in a colour change
based on the Griess assay [44,45]. The Griess assay is one of the most well-known methods for the
colorimetric detection of nitrite and has been used since 1879. A typical Griess assay relies on the
formation of diazonium salt due to the reaction of sulfanilamide with nitrite under acidic conditions. The
formed diazonium salt creates a coloured azo dye upon reacting with NED. Figure 3 shows a scheme
of the colorimetric detection of nitrite using the Griess reaction. Like all other colorimetric assays, a
spectrophotometer is needed for analysis which makes it unsuitable for on-site detection. Hence, there
is a need to develop a new POC analysis tool for colorimetric assays [46,47]. The proposed device allows
rapid detection of pH values and nitrite concentrations, and also a replication of the colorimetric assays
in a single unit.
A UV–vis, Evolution 60 s (Thermo Fischer Scientific, Madison, WI, USA) was used to determine the
absorption of nitrite. The same experimental conditions were used for the absorption determination
and during tests of the developed device. Water was used as a blank to generate the baseline. All
samples were measured at 540 nm, and a zero absorbance was set before any measurements were taken
................................................
Figure 2. A sample of colour values for the mixed pH indicator.
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mixture of methyl red and bromothymol blue
4
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5
2.4. Device calibration
Aqueous solutions of phosphate buffers of various known pH values and solutions of different nitrite
concentration were made to calibrate the device for pH measurement. Sodium phosphate monobasic
monohydrate and sodium phosphate dibasic were mixed to prepare phosphate buffer solutions with
varying pH values. All aqueous solutions were prepared using DI water. An Oakton pH meter (Oakton
Instruments, pH 510 series) was calibrated against three standard buffer solutions with three different
pH values (4.0, 7.0 and 10) and was then used for the pH determination.
The number of reference points to be stored was the primary parameter to be determined for
calibration. Several images of each sample were taken for calibration. The images were taken using the
integrated camera at a fixed distance of 1 cm, at room temperature, and under fixed and stable lighting
conditions. All images were taken and processed within 1 s, and the calibration data points were stored
in the device memory.
2.5. Colorimetric testing method
The portable device shown in figure 1 records an image of the coloured sample; the image information is
then processed and converted to the corresponding pH value or nitrite concentration by the developed
detection algorithm. The values are displayed on the touchscreen. To perform the colorimetric assay
for the determination of pH, 1 ml of a phosphate buffer solution of known pH value (prepared for
calibration) and different solutions with unknown values (for data validation) were mixed with 250 µl
of bromothymol blue and 50 µl of methyl red. All reagents were mixed thoroughly. A period of 5 s was
needed to produce the colour change in a 1 ml cuvette. To perform the nitrite colorimetric test, 1 ml
of each sodium nitrite concentration was transferred to a 1 ml cuvette. Then, 50 µl of 1% sulfanilamide
solution was added to the sample, and the cuvette was capped and mixed by inverting three times. Then,
50 µl of the 0.02% NED solution was added, and the cuvette was inverted three times again. The cuvette
was left at room temperature for 5 min to develop colour and was then inserted into the portable device
for measuring.
Once all the reagents were allowed to react and had produced a colour change, a picture of the
coloured sample was taken and processed. The detection process was carried out under controlled
lighting conditions at room temperature and using the exact conditions and steps followed for generating
the calibration curve. The HSV coordinates were extracted from the coloured image, analysed and
converted to the corresponding pH value or nitrite concentration by comparing the analyte data with
the calibration data using the developed algorithm.
................................................
of the nitrite concentrations. Samples were prepared in cuvettes and experiments were conducted at
room temperature.
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Figure 3. A schematic showing the formation of the azo compound upon the interaction of nitrite with the Griess reagent.
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Figure 4. The prototype GUI designed using the PyGameUI library for Python.
HD image
1024 × 768 px
ROI
100 × 100 px
normalized box filter
BGR to HSV
hue
saturation
value
Figure 5. Image processing pipeline performed using OpenCV.
2.6. Software
Software for the colorimetric device was written in Python. Support libraries used were OpenCV for
image processing, and Pygame and PygameUI for developing the custom graphical user interface (GUI),
allowing the selection of a particular, or all five cuvettes in the carousel. After capturing the images,
the nitrite concentration or the pH value of the sample is displayed on the screen (figure 4).
2.7. Image processing
The OpenCV library was used to perform image analysis [48] (figure 5). The original image of each
cuvette was captured at the full resolution of 1024 × 768 pixels (figure 6a). Then, a region of interest
(ROI) at the size of 100 × 100 pixels was obtained around the centre of the cuvette (figure 6b). A box filter
smoothening function (box blur) was applied to the ROI to obtain a uniform representation of the average
pixel colour value. The normalized box smoothening computes the value of each pixel as the mean of
its kernel neighbours. The average pixel value of the ROI is computed using 200 × 200 pixels which are
double the size of the ROI in both horizontal and vertical directions (figure 6c). In this way, the imageprocessing algorithm yields consistent results even with slight inconsistencies in the cuvette location or
external lighting. The ROI was converted from the red, green, blue (RGB) colour space to the HSV colour
space to achieve a linear relationship between the colour change and colour components.
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(a)
(b)
(c)
7
100
mixture of methyl red and
bromothymol blue
80
y = –2.135x4 + 49.564x3 – 417.29x2
+ 1524.1x – 2037.6
R2 = 0.998
h 60
40
20
0
pH
4.0
4
5
6
pH
7
8.0
8
Figure 7. The hue values as a function of pH values (ranging from 4 to 8) and the fitted polynomial curve.
3. Results
3.1. pH detection
3.1.1. Calibration curve
For the determination of pH values, the colorimetric test for each pH value was performed three times
under the same conditions using a mixture of bromothymol blue and methyl red. The period needed for
the detection was 5 s from the time the sample was inserted into the portable device until the image was
captured. The response of the mixed indicators was studied using phosphate buffer solutions of various
pH values ranging from 4 to 8. Figure 7 shows the change in the hue coordinate for the mixed indicator,
with a value ranging between 5 and 110. This wide range of values indicates that the hue coordinate can
provide a significant measurement with sufficient resolution for the pH value of each indicator solution.
A fourth order polynomial curve was fitted to the experimental data, enabling determination of the pH
value of an unknown solution from the hue value obtained using the device.
3.2. Nitrite detection
3.2.1. Detection limit and calibration curve
For the determination of the nitrite concentrations, the colorimetric test was repeated three times under
the same conditions mentioned in the previous sections. The formed azo dye from the reaction between
the nitrite and Griess reagents results in a colour change, from colourless to pink, then to red as the nitrite
concentration increased in the samples. The results also show that the (S) value of the image remains
constant above 1.8 mg l−1 of nitrite concentration in the sample (figure 8a).
The time needed for the detection was 5 min, from when the sample was mixed until the image
was captured. Owing to the colourless nature of the Griess solution used for the detection of the nitrite
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120
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Figure 6. Images in various stages of processing using the OpenCV library for Python. (a) Original image (1024 × 768 pixels), (b) ROI
(100 × 100 pixels) and (c) after application of 200 × 200-pixel box filter (100 × 100 pixels).
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(a)
8
200
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180
160
140
120
S 100
80
60
40
20
0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
nitrite concentrations mg
(b)
1.6
1.8
2.0
l–1
200
180
160
y = 100.06x + 5.8095
R2 = 0.9999
140
120
S 100
80
60
40
20
0
0.5
1.0
1.5
2.0
nitrite concentrations mg l–1
Figure 8. (a) The sensor response to different nitrite concentrations in the sample and (b) nitrite concentration calibration curve (line
fitted to the experimental S values obtained for different nitrite concentrations using Griess reagents).
concentration, the saturation coordinate (S) proved to be the most accurate and reliable coordinate for
measuring the colorimetric change, as it is proportional to the nitrite concentration in the sample. The
sensor response (S value) from different nitrite concentrations and the generated calibration curve are
shown in figure 8a and b, respectively. In this case, the calibration curve is a line, and the correlation
coefficient R2 was 0.9999. The minimum detection limit achieved for nitrite was 0.2 mg l−1 (figure 8a),
which is below the acceptable range for drinking water [49], which is 1 and 3 mg l−1 according to the
US Environmental Protection Agency [50] and World Health Organization (WHO) [51], respectively.
The experimental error bars in figures 7 and 8 were too small to be seen, confirming the high precision
of the data generated by the portable device.
3.3. Data validation
Additional experiments were carried out to examine the repeatability, accuracy and reliability of the
measurements using the portable device. For the pH data validation, five phosphate buffer solutions,
with different pH values, were prepared. The mixture of pH indicators (methyl red and bromothymol
blue) was used for the data validation. Phosphate buffer solution (1 ml) was mixed with 50 µl of methyl
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9
7.5
6.5
6.0
5.5
y = 0.993x + 0.116
R2 = 0.9998
5.0
4.5
4.0
4
5
6
pH meter value
7
8
Figure 9. A linear relationship between the pH values obtained using the pH meter and those obtained using the portable device.
Table 2. Comparison between the pH readings obtained using the pH meter and the portable device.
pH meter value
4
portable device measured pH value
4.1
.........................................................................................................................................................................................................................
5
5.04
6
6.08
7
7.07
8
8.05
.........................................................................................................................................................................................................................
.........................................................................................................................................................................................................................
.........................................................................................................................................................................................................................
.........................................................................................................................................................................................................................
Table 3. Comparison between the results of the nitrite solutions prepared against those obtained by the portable device.
nitrite concentration in sample (ppm)
the portable device measured nitrite concentration (ppm)
0.5
0.6
1.0
1.11
1.5
1.6
2.0
2.12
.........................................................................................................................................................................................................................
.........................................................................................................................................................................................................................
.........................................................................................................................................................................................................................
.........................................................................................................................................................................................................................
red and 250 µl of bromothymol blue to test the pH value. All tests were completed in a 1 ml cuvette.
Reagents were mixed thoroughly and placed in the device for 5 s, for the colour change to occur. All
tests were conducted three times. To perform the nitrite colorimetric test, the portable device was tested
against four random samples with different nitrite concentrations (0.5, 1.0, 1.5 and 2.0 mg l−1 ).
All reagents were allowed to react and produce a colour change before images of the samples
were taken and processed by the device. The detection process occurred under the same conditions
as those performed for the calibration process. Tables 2 and 3 summarize a comparison between the
data obtained using traditional analyses and those obtained by the portable device for the pH values
and nitrite concentrations, respectively. In general, the results are in good agreement, with maximum
errors occurring at the lowest pH values and nitrite concentrations. To have a better presentation of these
comparisons, the values listed in tables 2 and 3 are graphed using a unity line (figures 9 and 10). The data
generated by the device have a mean standard deviation value of 0.03 and 0.073, and correlation factor,
R2 , of 0.998 and 0.999 for the pH values and nitrite concentrations, respectively. These results confirm
that the portable device is accurate and reliable for the rapid detection of pH and nitrites.
In figures 8 and 10, the detection values of nitrite at a concentration of 0 mg l−1 were not measured
due to the colourless nature of the Griess solution used for the detection of the nitrite concentration.
................................................
7.0
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handheld device pH value
8.0
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10
1.5
y = 1.01x + 0.095
R2 = 0.9999
1.0
0.5
0
0.5
1.0
1.5
nitrite concentration in sample (mg
2.0
2.5
l–1)
Figure 10. A linear relationship between the concentrations of the prepared nitrite solutions and those obtained using the
portable device.
3.4. Nitrite measurement comparison
A comparison study was carried out between the produced data for nitrite detection by the developed
device and traditional UV–vis spectroscopy. Figure 11a,b shows a full spectrum of UV absorption and
the produced calibration curve of different nitrite concentrations, respectively.
Table 4 shows a comparison between the developed colorimetric device and the traditional UV–vis
for nitrite detection.
The comparison of the results and portability in table 4 shows that the developed portable colorimetric
device possesses a broad linear range and a low detection limit when compared with other methods
reported in the literature for the detection of nitrite. Furthermore, the device is simple to use and does
not require trained personnel to operate it. It should be noted that the price to manufacture our device
is significantly lower than that of the other methods, approximately four to five times cheaper, which is
a significant advantage for the overall proliferation of the technology. Also, the device showed another
significant advantage over the UV–vis spectroscopy and HPLC, portability, which makes the device more
suitable for online and remote-area detection.
4. Discussion
The use of smartphones for sensing colour change is often subject to different light conditions, resulting
in a high possibility of interference from ambient light. This significantly limits the use of these devices in
sensing technologies and for POC applications. Hence, there is a need to overcome this obstacle using (i)
a stable and reliable external source of light for all measurements and (ii) a designated means of keeping
the distance between the samples and camera constant [38,39]. Also, spectrophotometric absorption
is one of the widely used technologies for detection of colorimetric assays. However, this instrument
is bulky and expensive, and needs trained personnel to run it, which makes it inadequate for POC
applications [55]. As a result, developing a versatile, low-cost, rapid and portable colorimetric device
is important and needed for on-site detection.
This work demonstrates the development of a low-cost and versatile portable colorimetric detection
device for on-site detection and potentially for POC diagnostics. Three-dimensional printing technology
enabled fast prototyping for this project. The three-dimensional-printed design was customized to (i)
accommodate the rotating carousel mounted on a motor to manipulate five separate cuvettes around the
camera for multiple sample analysis; (ii) contain an LED as the external light source that controls the
light conditions during image capture; (iii) rapidly analyse the colour change in 8-megapixel resolution
images and (iv) contain onboard electronics that provide fast and robust detection of the HSV coordinates
of the captured images. While using higher quality components, specifically a higher resolution camera,
may slightly increase precision, upgrading hardware should not significantly change data acquired.
Additionally, the developed device is battery-powered, wirelessly connectable and equipped with a
Raspberry Pi computer for data processing, storage and exchange to make it suitable for POC diagnostics.
................................................
2.0
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handheld measureed nitrite
concentration (mg l–1)
2.5
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absorbance
1.0
................................................
1.2
11
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(a)
nitrite concentrations
(mg l–1)
2.0
1.8
1.6
increase in absorbance at
540 nm
1.4
0.8
1.2
1.0
0.6
0.8
0.4
0.6
0.2
0.4
0.2
0
410
(b)
460
510
wavelength (nm)
0.1
560
blank
1.0
0.9
absorbance at 540 nm
0.8
0.7
y = 0.5631x + 0.0072
R2 = 0.9969
0.6
0.5
0.4
0.3
0.2
0.1
0
0.5
1.0
nitrite concentration (mg l–1)
1.5
Figure 11. UV–vis measurements for nitrite concentrations (a) UV–vis spectrum for the determination of different nitrite concentration
and (b) nitrite concentration calibration curve (line fitted to the experimental absorbance values at 540 nm obtained for different nitrite
concentrations using Griess reagents).
Table 4. Comparison of the data obtained by UV–vis spectroscopy, the developed device and other methods reported in the literature.
method
linear detection range (mg l−1 )
limit of detection (mg l−1 )
portability
ref.
UV–vis
0.1–1.6
0.1
no
this work
HPLC–UV
0.1–100.0
0.05
no
[52]
no
[53]
.........................................................................................................................................................................................................................
.........................................................................................................................................................................................................................
−3
Spectrofluorometric
0.005–0.500
2.5 × 10
Spectrophotometric ion-pair HPLC
0.01–0.1
0.1
no
[54]
our device
0.4–1.6
0.2
yes
this work
.........................................................................................................................................................................................................................
.........................................................................................................................................................................................................................
.........................................................................................................................................................................................................................
It could be envisioned that outputs for data may be interfaced with a smartphone in the future, to further
increase accessibility without comprising accuracy or precision [56].
5. Conclusion
The developed colorimetric device shows several advantages compared with the traditional method for
colorimetric detection, using UV–vis spectroscopy. First, small and inexpensive off-the-shelf components
Downloaded from http://rsos.royalsocietypublishing.org/ on March 23, 2018
5061/dryad.pj475) [58].
Author contribution. G.S.L. originated the idea and designed the experiments, performed the experiments, analysed the
data, selected reagents/materials/analysis tools, wrote the manuscript, prepared figures and tables, and reviewed
drafts of the manuscript. E.N. designed and developed the hardware and software of the device, wrote the
corresponding part of the manuscript and reviewed drafts of the manuscript. J.K. performed parts of the experiments
and reviewed drafts of the manuscript. M.H. contributed to the development of the device, commented on the design
of the experiment and reviewed the manuscript. H.N. reviewed and revised the design, reviewed the manuscript and
oversaw the project.
Competing interests. We have no competing interests.
Funding. This work was supported by Natural Sciences and Engineering Research Council of Canada Discovery grants
(341873 M.H. and 341887 H.N.).
Acknowledgements. The authors thank Kurt Yesilcimen for his help in the three-dimensional modelling and printing
the device.
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