Distance and Color Change Based Hydrogel Sensor for Visual

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Distance and Color Change Based Hydrogel Sensor for Visual Quantitative Determination of Buffer Concentrations Renjie Wang, Xinfeng Du, Jingying Zhai, and Xiaojiang Xie ACS Sens., Just Accepted Manuscript • DOI: 10.1021/acssensors.9b00186 • Publication Date (Web): 21 Mar 2019 Downloaded from http://pubs.acs.org on March 22, 2019

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ACS Sensors

Distance and Color Change Based Hydrogel Sensor for Visual Quantitative Determination of Buffer Concentrations Renjie Wang, Xinfeng Du, Jingying Zhai, and Xiaojiang Xie* Department of Chemistry, Southern University of Science and Technology, Shenzhen, 518055, China *

Email: [email protected]

Abstract. We present here an innovative platform for the determination of pH buffer capacity based on FITC-dextran loaded hydrogels. Optical signals from the pH sensitive hydrogels were analyzed by simple parameters including distance and color change. The methodology was validated on five different buffer systems and exhibited wide linearity (0.1 to 100 mM), good batch-to-batch reproducibility, high versatility and resistance to background ionic strength changes. Experimental results also fit well with theoretical model based on numerical simulation. Preliminary application in carbonate alkalinity determination of seawater was proved very successful. This hydrogel buffer concentration sensor is fundamentally different from conventional acid-base titrations, brings minimum perturbation to samples, and shows great potential in real applications. Key words: buffer capacity, distance-based, visual detection, alkalinity, acidity, hydrogel, agarose, fluorescence pH buffers are ubiquitous in natural environments (e.g., sea

coulometric way.8 Similarly, electrochemical in situ splitting of

water), biological fluids (e.g., blood), and research laboratories.

water to form H+ on graphite electrodes was also used for online

Buffer solutions help maintain a constant pH (or nearly so)

buffer capacity measurement of wastewater.9 More recently,

against the addition of acids or bases. The amounts of acid or base

Bakker and co-workers presented a series of innovative

that can be added without causing a significant pH change rely

approaches to measure alkalinity, acidity, and pH using

on the buffer concentrations, which increases with the buffering

membrane electrodes which pump defined amounts of protons

species

(conjugate

weak

acid-base

pairs).

1

The

buffer

under

electrochemical

control.10-12

In

these

methods,

concentrations of oceans provide high value for marine biology,

conventional titrant was replaced by electrochemically generated

climate change, water treatment and more globally, to what extent

H+ polarization which was quantifiable through coulometry.

could the oceans cope with anthropogenic CO2 in the atmosphere,

These methods indeed require only small amounts of samples and

preventing the pH of seawater from decreasing dramatically.

2-3

may become suitable for submersible detection. In comparison,

Acid-base titrations are routinely used to measure environmental

new optical methods for buffer capacity measurements were

buffer concentrations to obtain the total alkalinity and total

rarely reported. Mistlberger and co-workers previously proposed

acidity information.4-5 However, stepwise addition of strong acids

a

photoswitchable

hydrogel

containing

a 13

photochromic

or bases to titrate the sample while recording the pH is time-

compound spiropyran and cation exchanger.

consuming, require strictly standardized titrants, and typically

measurements were activated by illuminating the hydrogel with

need a large volume of sample. Direct addition of strong acids or

ultraviolet light to convert spiropyran to a much more basic

bases to the sample sometimes could be very invasive, giving rise

merocyanine form. The advantage is that this is a photo-triggered

to undesired chemical transformations. In some cases, the results

active sensor. However, the ion-exchange nature makes the

from titrations also required mathematical algorithm for

measurements dependent on background ionic strength.

corrections on pH observations due to residual liquid junction

Therefore, it is clear that although new methodologies for buffer

4, 6

capacity determination is in high demand, current technology still

Therefore, innovation in buffer capacity detection methodologies

revolves around the acid-base titration while optical sensing

that is titration free is in high demand.

methods remain largely unexplored.

Some advances for the buffer concentration measurements have

On the other hand, portable sensors with visual readout

potentials that could cause an error of up to 0.1 pH unit.

Buffer capacity

Apart from automated titrators that

(including color and distance) are becoming more and more

titrate with conventional acid/base solutions, electrolysis of water

welcome for onsite and in situ applications.14-20 The cost of the

to produce either H+ or OH- in ISFET devices were proposed by

sensors will be significantly reduced if the sensor signals are

van der Linden and co-workers to determine buffer capacity in a

acquired in instrumentation-free ways. Visual readouts have been

appeared in recent years.

5, 7-13

1

ACS Paragon Plus Environment

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reported for a number of sensors which monitored signals

(purified by Milli-Q Integral 5). Standard solutions of HCl and

including the change of color, distance, volume, and pressure.20-

NaOH (1 M) were used to adjust the buffers to the target pH

25

values.

Previously, we have introduced ion-selective hydrogels that +

+

2+

2+ 26-

contain highly selective ionophores for Na , K , Ca , and Pb . 27

Preparation of the FITC-dextran hydrogels. First of all, 2

The hydrogel interacted dynamically with the sample and the

mg/mL FITC-Dextran stock solution was prepared in deionized

results could be conveniently obtained by analyzing the change

water. To prepare the hydrogel, agarose powder at 1% weight

of distance and color (within the gel or in the sample solution).

percentage was mixed with deionized water and heated up until

The additional function of the hydrogels as a filter also enabled

dissolution. Then, the FITC-dextran stock solution was mixed

+

+

the detection of Na and K in human blood and serum.

with the agarose solution with a volume ratio of 1:10. For UV-

In this work, we present for the first time the determination of

visible absorption measurements, 1 mL of the mixture was placed

buffer concentrations using pH sensitive hydrogels which interact

in a disposable PMMA cuvette (4.5 mL), sealed with para film,

with the sample in non-equilibrium mode. The buffer

and put flat on a table so that the hydrogel was cast on the wall of

concentrations could be determined by observing how fast the

the cuvette after cooling to room temperature. Similarly, for

color of the hydrogel changes, and alternatively, the change of

distance analysis, the hydrogel solution was also pipetted into a

distance. Buffer systems including the carbonates, Tris-HCl, and

cuvette, and the cuvette was placed upside down on a flat glass

Mes-NaOH were evaluated as models to test if the method is

slide to form a flat contact interface for the sample. Without

general and universal. Preliminary application was demonstrated

specifying, the hydrogels were all prepared with deionized water.

with carbonate alkalinity measurements in seawater sample.

Instrumentation and Measurements. UV-visible absorption

Characterization of the hydrogel sensor under different ionic

spectra were recorded on an absorption spectrometer (Evolution

strength also showed agreeable results.

220, Thermo Fisher Scientific). Hydrogels were cast on the

EXPERIMENTAL SECTION

cuvette wall as mentioned above, and the absorbance at 490nm was monitored in kinetic measurement mode. The sample volume

Reagents. Fluorescein isothiocyanate–dextran (FITC-dextran,

was typically 1 mL. The pH response of FITC-Dextran was

molecular weight 70000 Dalton), agarose (low gelling temperature),

evaluated in a universal buffer solution (2.5 mM of citric acid,

2-amino-2-(hydroxymethyl)-1,3-propanediol

boric acid, and NaH2PO4), with the pH gradually adjusted with 1

(Tris), Mes hydrate, sodium hydroxide, sodium bicarbonate, and sodium

chloride

were

purchased

from

M NaOH solution. Fluorescence spectra was recorded on a

Sigma-Aldrich.

fluorescence spectrometer (Fluorolog-3, Horiba). For laser

Hydrochloric acid was purchased from Dongguan Dongjiang

scanning fluorescence confocal imaging, hydrogel samples were

Chemical Reagent in China. Sea water sample was collected from

cast on microscope slides and covered with glass slips. The

the Shenzhen Bay area by Dr. Chuanlun Zhang at the department

images were obtained on a Nikon A1R confocal microscope with

of ocean science and engineering in Southern University of

the standard FITC filter cube, and laser line at 488 nm was

Science and Technology.

selected for the excitation. For distance analysis, the hydrogels

Preparation of buffer. All buffer solutions were prepared by

were illuminated with a portable UV lamp (365 nm) while

dissolving appropriate amounts of salts into deionized water

(a)

Buffer Solution

(b)

FITC-Dextran Buffering Species

pH Responsive Hydrogel

t0

t1

t2

t3

Figure 1. (a) An illustration of the sensing principle: a time-dependent color change resulting from diffusion of buffering species from the aqueous solution to the pH responsive hydrogel. (b) Laser scanning confocal fluorescence microscopy image of the hydrogel. Scale bar: 25 µm. 2


Page 3 of 8

∂C ( x ) ∂J ( x ) = ∂t ∂x

pictures or videos were captured with a digital camera (Canon 5D Mark IV). Distance was quantified from the histogram profiles

(2)

C x ( t + Δt ) − C x ( t ) J x−1/x ( t ) − J x/x+1 ( t ) = (3) Δt d position x to position x+1, Di the diffusion coefficient, and C(x,t)

constructed using the software ImageJ. Seawater samples were titrated manually (without dilution) with 10 mM hydrochloric acid to pH 3.0 or directly analyzed with the hydrogels containing

the concentration of HA- at position x and time step t. Notice that

FITC-dextran in deionized water in absorbance mode, using

the diffusion coefficients in the hydrogel was assumed only half

NaHCO3 solutions from 1 to 5 mM at pH 8.1 as calibration

the value in the solution. The continuity equation of Fick’s second

standards.

law (Eqn. 2) was rewritten as Eqn. 3, where ∆t (25 ms) is the

RESULTS AND DISCUSSION

length of the time increment. Solving Eqn. 1 and 3 resulted in the

(w/w) of agarose and FITC-dextran (0.18 mg/mL). Agarose

expression as shown in Eqn. 4 which was then iterated in Δt C x ( t + Δt ) = C x ( t ) + 2 Di {C ( x − 1,t ) − 2C ( x,t ) + C ( x + 1,t )} (4) d Wolfram Mathematica. After each iteration, assuming that acid-

hydrogels were previously used for sensing purposes.26, 28 With

base equilibrium is very fast, the pH value, aH+ (x,t), at each

simple heating and cooling, the agarose with low gelling point

position can be calculated according to the dissociation equilibria

used here made the hydrogel formation process very easy and

of H2A as expressed in Eqn. 5, where Ka1 and Ka2 represented the

Figure 1a shows the schematic illustration of the sensing concept and the composition of the hydrogel. The hydrogels contained 1%

K a1K w + K a1K a2C ( x,t ) (5) K a1 + C ( x,t ) first and second acidity constant of H2A, respectively, and Kw is

convenient (see Experimental Section for details). FITC-dextran

aH + ( x,t ) =

(molecular weight 70000 Dalton) is a pH sensitive fluorescent polysaccharide. The pH response in light absorption and

the self-ionization constant of water.1 The results from the model

fluorescence mode was characterized to show an apparent pKa of

are shown in Figure 2 by assuming a sigmoidal hydrogel

6.4 (Figure S1). Laser scanning confocal fluorescence microscopy imaging confirmed quite even distribution of FITC-

(a)

(d)

0

��

dextran inside the agarose hydrogel (Figure 1b). Although FITCx

� �� / � �

difficult to escape from the hydrogel network. Leakage of FITC-

[��] / ��

��

dextran in to contacting aqueous solution was negligible, as confirmed by spectroscopic monitoring of the solution phase absorption spectra (Figure S2). The buffer concentration sensing

0s

� � � � � -�

180 s

the hydrogel. When the hydrogels are exposed to pH buffer solutions, the buffering species would diffuse into the hydrogel and change the local pH. The change of local pH is then reflected by FITC-dextran (in color, absorbance, and fluorescence). As

hydrogel

�� 0s �� solution -�� -��

60 s 90 s 120 s 150 s 180 s 210 s 240 s

��

��

solution

(c)

principle is based on diffusion controlled gradual pH change in

30 s

��

30 s 60 s 90 s 120 s 150 s 180 s 210 s 240 s

(b)

-� � � �� / ��

��

�

�� / ��

(e)

180 s

�� / ��

��

��

hydrogel

��

��

dextran is water-soluble, once trapped in the hydrogel it becomes

��

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ACS Sensors

��

�

�

�

�

��

��

� (�)

buffer concentration goes higher, the flux of the buffering species

Figure 2. (a) Simulated fluorescence propagation of the hydrogel

becomes bigger and the optical signals change faster.

at different time points after contacting the buffer solution (10

A theoretical model was developed based on amphoteric buffers

mM HA-) as a consequence of one-dimenssional diffusion along

(mimicking NaHCO3 buffer) which in general contains HA-. The

direction x. (b) Temporal evolution of the buffer species

local pH within the hydrogel and the optical sensor readout was

concentration profile from 0 to 180s. (c) Temporal evolution of

simulated using the finite element analysis method. Here, the

the pH profile from 0 to 180s. (d) First derivative of the hydrogel

hydrogel and the solution in contact were divided into 600

fluorescence intensity profile along direction x at different time

C ( x,t ) − C ( x + 1,t ) (1) d positions with equal step (d) of 20 µm. Fick’s first law was then

points as indicated. The peak position indicates how far the

transformed to Eqn. 1 by assuming linear gradients between the

equals 10 -5 cm 2/s in solution and 5×10-6 cm2/s in hydrogel, Ka1

J x/x+1 ( t ) = Di

fluorescence propagated. (e) Theoretical absorbance change of the hydrogel as a function of square root of time. Parameters: D i

-

adjacent positions. Jx/x+1(t) represents the flux of HA from

equals 4.3×10-7, Ka2 equals 4.8×10-11, Kw equals 10-14. 3


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Page 4 of 8

fluorescence response to pH which was indeed experimentally

hydrogel after exposing to NaHCO3 buffer. Clearly, absorbance

observed (Figure S1). Here, the buffer concentration was 10 mM

changed more rapidly for higher buffer concentrations. The

with the rest of the parameters shown in the figure caption. Figure

absorbance changing rate, designated here as the “slope”, also

2a demonstrates the propagation of the fluorescence signal

changed linearly as a function of logarithmic buffer concentration.

propagation along the direction of the flux (x direction). The

A good linearity from 0.1 to 100 mM was observed, which was a

brighter the green color, the stronger the fluorescence and the

lot wider compared with previously reported electrochemical

higher the local pH. The time evolutions of the buffer

methods based on chronopotentiometry.12 However, as the

concentration and the pH profile are shown in Figure 2b and 2c,

concentration becomes smaller, the signal gets noisier, and

respectively, where the aqueous sample located at positions

change in absorbance (∆A) also gets smaller, indicating a limit

below 0 mm while the hydrogel located at positions above 0 mm.

of detection around 0.1 mM. Concentrations lower than 0.1 mM

The signal change (in both fluorescence and absorbance) is

were not attempted because of the poor buffer effect and unstable

essentially determined by how far the buffer front has travelled.

pH.

The front of the buffer in the hydrogel was identified by the

Besides NaHCO3 buffer solutions, Tris-HCl and Mes-NaOH

maximum of the first derivative of the fluorescence intensity over

buffers were also evaluated at different pH values. Figure 4a, 4b

distance (dFL/dx). The peak maxima positions in Figure 2d

and 4c show the absorbance responses of the hydrogels towards

corresponded to the fronts of the buffer species where the

various Tris-HCl buffer concentrations while the responses

fluorescence changed abruptly. Clearly, the spatial progression of the optical signal change is time dependent and after 3 minutes, a color change distance of around 1.6 mm was predicted. This prediction agreed very well with our experimental observations (vide infra). Plotting the peak maximum positions against the square root of time resulted in a linearly relationship, which is not surprising considering common diffusion characteristics. And following Beer’s law, the change of absorbance along direction x depended linearly on the square root of time (Figure 2e). Indeed, the linear change in absorbance over square root of time was experimentally observed. Here, the UV-visible absorbance change at 490 nm was recorded with the light path in parallel with x direction. Figure 3a and b show the absorbance change of the

Figure 4. First row: Temporal evolutions of absorbance changes of the hydrogels at 490 nm after exposing to pH 7.4 (a) and pH 8.1 (b) Tris-HCl buffer solutions with various concentrations as indicated. (c) Corresponding calibration curves by plotting the slope values against logarithmic Tris concentrations.

Second

row:

Temporal

evolutions

of

absorbance changes of the hydrogels at 490 nm after exposing

Figure 3. (a) Temporal evolutions of absorbance changes of

to pH 5.5 Mes-NaOH buffer solutions with various

the hydrogels at 490 nm after exposing to pH 8.1 NaHCO3

concentrations as indicated. The hydrogels contained deionized

buffer solutions with various concentrations as indicated. (b)

water for (d) and 0.1 mM NaOH for (e). (f) Corresponding

Corresponding calibration curve by plotting the rate of

calibration curves by plotting the slope values against

absorbance change (slope) against logarithmic concentrations

logarithmic Mes concentrations. Error bars represent standard

of NaHCO3 solutions. Error bars represent standard deviations

deviations from 3 measurements.

from 3 measurements. 4


towards Mes-NaOH are shown in Figure 4d, 4e and 4f. For both

(a)

��

buffer systems, linear calibration curves were obtained in a range

��

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ACS Sensors

from 1 to 100 mM. In addition, the performance of the hydrogels for acetate buffers at pH 3.6 and phosphate buffers at pH 10.0

��

were also evaluated. The results shown in Figure S3 also

(b) Mes-NaOH

Tris-HCl

��

Page 5 of 8

��

�� (��)

��

��

��

��

�� (��)

��

exhibited good linear responses from 1 mM to 100 mM.

Figure 6. Image analysis on the pictures in which the hydrogels

For a significant signal change to be observed, the initial pH of

were exposed to 10 mM pH 8.1 Tris-HCl (a) or pH 5.5 Mes-

the hydrogel and the buffer should be sufficiently different. The

NaOH (b) for 3 minutes. Hydrogels contained deionized water

Mes-NaOH buffer solutions evaluated here were acidic (pH 5.5),

and 0.1 mM NaOH for (a) and (b), respectively. Dotted vertical

while typically the hydrogel prepared with deionized water has a

lines indicate the transition positions.

pH around 6.0. As shown in Figure 4d, when the hydrogel

all, distance even could be analyzed with naked eyes, and more

contained only FITC-dextran and deionized water, the

accurately, by image analysis on the pictures of the hydrogels.

absorbance at 490 nm started to decrease. However, the change

Here, the pictures of the hydrogels in disposable plastic cuvettes

was quite small and the signal was relatively noisy. Although the

were captured from the side view with a digital camera. To help

linear decrease of absorbance against the square root of time was

enhance the contrast, the hydrogels were illuminated with a UV

still evident, the slope change was only marginally satisfactory.

lamp (365 nm). The pictures in Figure 5a was recorded 3 minutes

The obvious solution to improve the signal-to-noise ratio and

after adding the buffer solutions (Tris-HCl). The boundaries of

hence the sensitivity was to increase the pH of the hydrogel. To

color were readily identifiable. In agreement with our model, a

test this hypothesis, the hydrogel was load with NaOH (0.1 mM).

linear relationship was observed between the distance of color

As shown in Figure 4e, indeed, both the absorbance change and

change and logarithmic buffer concentrations (between 1 and 100

the signal-to-noise ratio were improved. In addition, as shown in

mM). The results also agreed well with the results from kinetic

Figure 4f and 4c, the sensitivity of the method increases upon

absorbance measurements. The distance change shown in Figure

widening the pH gap between the hydrogel and the sample buffer.

5a was indeed not very big. One way to enhance the difference is

The results discussed above verified that the method was indeed

to wait longer. As shown in Figure S4, the distance increased to

highly promising in determination of buffer concentrations.

around 6 mm after 30 minutes.

However, monitoring the absorbance change onsite still could be

Here, the distances in Figure 5b were obtained with ImageJ

demanding due to, for instance, the requirement of at least a

analysis. The standard deviations for the distance determination

miniature spectrometer. A more convenient alternative is to

were quite small, as shown below in Table 1 for 3 measurements.

measure the distance of the fluorescence or color change. After

Boundary identification with naked eyes were also possible but could be less accurate because the results may vary between individuals. To help measure more accurately the distance change, image analyses using the software ImageJ was employed. ImageJ helped produce the histogram profiles (a function of gray values against pixel positions) of the picture in any selected region. The edge of the hydrogel and the front of the bright green color (or dark green color, in the case of Mes-NaOH) were identified at the transition points. Figure 6 shows the distance analyses for two images captured 3 min after addition of buffers. Figure 6a and 6b correspond to 10 mM Tris-HCl buffer (pH 8.1) and 10 mM Mes-

Figure 5. (a) Pictures of the hydrogels 3 minutes after adding

NaOH buffer (pH 5.5), respectively. The black region represents

on

various

the buffer solution. In both cases, the gray values exhibited two

concentrations as indicated. (b) Corresponding calibration line

transition points (indicated by the vertical lines in the figures)

by plotting the vertical length (distance) of the brighter green

which help to accurately locate the boundaries.

the

top

Tris-HCl

buffer

solutions

with

color against logarithmic buffer concentrations. 5


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Page 6 of 8

Table 1. Comparison of distance changes on the hydrogels

linear response range from 0.1 mM to 100 mM. Compared with

exposed to buffer solutions with and without additional salt

conventional acid-base titrations, there is no need of manually

background.

adding reagents to the sample. Typically, 1 mL of sample volume

Buffers

Distance (mm)

Tris-HCl (10 mM)

Tris-HCl +

0.1 M NaCl

MesNaOH

(10 mM)

MesNaOH +

0.1 M NaCl

NaHCO3 (5 mM)

1.64±0.15 1.62±0.13 1.63±0.18 1.64±0.21 1.42±0.02

was sufficient for the experiments and potentially, with proper

NaHCO3 +

engineering the volume could be further reduced. Experimental

0.1 M NaCl 0.01M CaCl2 0.01M MgCl2

linear optical responses of the sensor agreed very well with theoretical expectations modeled with numerical simulations.

1.40±0.07

Real samples (such as seawater) may contain a complex

Together, these findings laid a good foundation for a new family

background including various ionic strength. Influence of sample

of user-friendly titration-free optical buffer concentration sensors

ionic strength was therefore, an important criterion of sensor

that holds big development prospects.

robustness. Here, the sensor responses to buffer solutions (as

ASSOCIATED CONTENT

listed in Table 1) were evaluated in distance response mode. For

Supporting Information

comparison, one set of the samples contained a salt background

Additional information as noted in the text include: pH response

(including monovalent and divalent ions) while the other not. No

of

significant differences the change of distance in 3 min were

calibration for seawater analysis. This material is available free

observed between the two sets of buffer solutions. The results

of charge via the Internet at http://pubs.acs.org.

indicated that the hydrogel sensor has a high resistance to

FITC-dextran,

leakage

evaluation,

hydrogel

images,

AUTHOR INFORMATION

background ionic strength change, which is a big advantage

Corresponding Author

compared with previous spiropyran-based system. As a

* Email: [email protected]

preliminary application, we attempted the carbonate alkalinity

ORCID

determination in seawater using the hydrogels. Seawater samples

Xiaojiang Xie: 0000-0003-2629-8362

were collected from Shenzhen Bay area into sealed bottles. A

Notes

series of NaHCO3 solutions with different concentrations at pH

The authors declare no competing financial interest.

8.1(which equals the seawater pH) were prepared as standard

ACKNOWLEDGEMENTS

solutions and quantitative analysis was obtained according to external calibration curves (Figure S5). The results from three

The authors thank the National Natural Science Foundation of

parallel experiments led to a carbonate buffer concentration of

China and the Shenzhen Municipal Science and Technology

2.9±0.2 mM. The same sample was also titrated with 10 mM HCl

Innovation Council for financial support.

solution to pH 3.0 (monitored using a pH meter) and gave a very

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Buffer Capacity

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Distance and Color Change Based Hydrogel Sensor for Visual

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