A visual volumetric hydrogel sensor enables quantitative and sensitive detection of copper ions

Article abstract of DOI:10.1039/c5cc00744e

We propose a visual volumetric sensor with 5,6-dicarboxylic fluorescein cross-linked amine-functionalized polyacrylamide hydrogel. The sensor undergoes volume response to Cu²⁺ ions at the μM level, which enables naked-eye quantitative detection by reading the graduation on a pipette.

Full text of DOI:10.1039/c5cc00744e

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A visual volumetric hydrogel sensor enables
quantitative and sensitive detection of
Cite this:DOI: 10.1039/c5cc00744e
copper ions†
ab
Received 26th January 2015,
Accepted 1st April 2015
a
c
a
a
a
Rui Wu, Shenghai Zhang, Jitong Lyu, Fang Lu, Xuanfeng Yue* and Jiagen Lv*
DOI: 10.1039/c5cc00744e
www.rsc.org/chemcomm
We propose a visual volumetric sensor with 5,6-dicarboxylic fluorescein
Compared with the variation in optical properties, the variation
cross-linked amine-functionalized polyacrylamide hydrogel. The in volume of liquid or fluid can be straightforwardly measured by
2+
sensor undergoes volume response to Cu ions at the lM level, reading the graduations on an appropriate container, for example,
which enables naked-eye quantitative detection by reading the a thermometer. Being aware of this, Qin’s group and Yang’s group
graduation on a pipette.
successively reported an elegant bar-chart chip design for visual
quantitative sensing. Once the analyte triggered the catalytic
11
Because of advantages such as being instrument-free and perhaps decomposition of hydrogen peroxide, a large amount of oxygen is
also in line with the human habits of ‘‘seeing is believing’’, naked- produced and collected to move an ink column, where the contents
eye readout chemical sensors are drawing increasing attraction in of the analyte are translated into volumetric signals. Beyond the
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many areas and disciplines. Most of the reported visual sensors great potential for point of care diagnostics for visual sensitivity
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are based on the notable changes in color indicating the presence analysis of ions, molecules and even cells, their studies proved
of a specific chemical species. Imaginative and sensible colori- the concept and feasibility of developing volumetric sensors for
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metric reagents such as nanoparticles, dyes, and polymers
naked-eye quantitative detection. Regarded as one of the most
have been introduced into the designs of analyte triggered visual appropriate materials for chemical and bio-chemical sensing,
sensors. Stimuli-responsive hydrogels have also attracted parti- responsive swelling/shrinking of hydrogel to various analytes has
12a,b
cular attention for their ‘‘smart’’ phase-transition in response to been well acknowledged.
Most of quantitative (or potentially
chemical or physical changes. Recent years have seen remark- quantitative) hydrogel sensing designs are developed by character-
able efforts made to develop visual hydrogel sensors for many izing the analyte stimulated variation in physical properties of
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important analytes, including nucleotides, lectin concanavalin color and light absorbance, light emission, resonance frequency,
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A, glucose, nitric oxide, melamine, metal ions, etc. These light diffraction and refraction rather than of volume, while in
sensors with straightforward designs are directly working devices many cases the responsive volume changes to analyte have been
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which provide simple, portable and sensitive detection without noticed. For example, Lin et al. reported a color visualizing copper
requirement of instrument or power source. However, in con- ion sensor with the DNAzyme and substrate incorporated hydrogel,
trast to the widely available qualitative and semi-quantitative where the proportional relationship between the size change of the
12c
detection, visual hydrogel sensors for quantitative detection hydrogel and the copper concentration was observed. All these
remain significant challenges.
studies imply the potential of developing volume readout hydrogel
sensors for quantitative sensing. The challenge is the difficulty of
measuring hydrogel volume changes with sufficient precision.
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And one step further, if the hydrogel volume changes can be
visually measured with high resolution, a quantitative volumetric
sensor can be expected. Inspired by such potential, we proposed a
a
Key Laboratory of Analytical Chemistry for Life Science of Shaanxi Province,
School of Chemistry and Chemical Engineering, Shaanxi Normal University, Xi’an,
7
10119, P. R. China. E-mail: lvjiagen@snnu.edu.cn, xfyue@snnu.edu.cn;
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volumetric sensor design for metal ion sensing. By filling Cu
Fax: +86 29 81530727
b
c
College of Chemical and Environment Science, Shaanxi University of Technology,
Hanzhong, 723001, P. R. China
stimuli-responsive hydrogel particles into a graduated pipette and
subsequently reading the volume response of hydrogel, quantitative
College of Chemical Engineering, Beijing University of Chemical Technology,
Beijing 100029, P. R. China
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detection of Cu at the mM level is readily accomplished.
The working principle of our hydrogel sensor is summarized
in Scheme 1A. Via Hoffman degradation, 19.4 percent of amide
groups in linear PAM were converted into amine groups. 5,6-DCF
†
Electronic supplementary information (ESI) available: Synthesis and character-
ization, responsive kinetics, the chelate structure, reproducibility, the use of a
scale bar, and sensor recovery. See DOI: 10.1039/c5cc00744e
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For such constructed device, the visual readout accuracy of volume
variation is estimated to be Æ20 mL. That is, for any volume
response exceeding 400 mL, the readout error is theoretically less
than 5%. Such precision is generally acceptable for quantitative
detections. Copper ions, at as low as the mM concentration level,
are found to trigger distinct and proportional volume responses
14
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(
Scheme 1B). According to the literature, it is supposed that Cu
ions form the heteroligand chelates with amine and carboxylic
groups in hydrogel, which brings about microscopic interchain
aggregation and consequent macroscopic volume shrinkage
coupled with water drainage.
To verify the assumption above, TEM and SEM investigations on
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the hydrogel micromorphology in the absence and presence of Cu
were conducted. A less 5,6-DCF cross-linked hydrogel was specifi-
cally synthesized for TEM investigation. As shown in Fig. 1A, the
water swollen and freeze-dried hydrogel exhibits a dendrimer-
like structure. In contrast, upon addition of 1.0 mM Cu , the
disappearance of dendrimer branches and the appearance of
Scheme 1 (A) The working principle of 5,6-dicarboxylic fluorescein (5,6-DCF)
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cross-linked amine-functionalized polyacrylamide (PAM) hydrogel for volu-
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metric response to Cu . (B) Photographic images of the hydrogel sensor
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and its responses to Cu ranging from 0 to 500 mM (figures in red), which cube-like blocks suggest the aggregation of the adjacent PAM chains
can be quantitatively distinguished by the naked eye. Borate saline buffer of
(Fig. 1a). As for the SEM investigation, the water swollen and freeze-
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mM pH = 7.4 was used to control the acidity as well as the ion strength.
dried hydrogel displays an irregular pore shape (with variable sizes
ranging from 15 mm to 50 mm) and an interconnected 3D network
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was used as the cross-linker as well as the dye to form the bright structure (Fig. 1B). Upon addition of 0.1 mM Cu , the hydrogel
yellow–green 3D network through dehydration between the dramatically shrinks its porous network and ends in a structure
amine group and the carboxylic group (S2, ESI†). The resultant with a smooth outer surface (Fig. 1b). Macroscopically, the hydrogel
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hydrogel is abundant in amine and carboxylic groups (S3, ESI†). exhibits drastic shrinkage in the presence of Cu . Such a SEM result
The hydrogel swelling capacity is of significance to achieve the is consistent with what we have observed in TEM investigation.
high resolution of volume readout, as well as the sufficient In addition, TEM and SEM results obtained also explain the
precision and sensitivity. To meet the need of the following phenomenon that smaller hydrogel particles perform better
2
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sensor construction, the hydrogel swelling capacity was deter- sensitivity to Cu . For bigger hydrogel particles, the initial
mined as the apparent swelling rate by measuring the accumu- chelation taking place on their surface leads to the formation
lation volumes of dried particles and fully swollen particles, of dense shells, and thus, the disappearance of micro-channels
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respectively. Experimental results found that the dried hydrogel kinetically prevents Cu from diffusing into the inner network.
particles (ground into 80–120 mesh) may swell in apparent In our sensor, each particle is an independent sensing element;
volume by about 70 times and 67 times with respect to pure the visually observed volume response is virtually the integration
water and 2 mM borate buffer (S4, ESI†). In our primary sensor of shrinkages of many particles. Consequently, the signal ampli-
construction, hydrogel particles were thoroughly immersed in fication and highly sensitive detection can be expected.
borate saline buffer, and the fully swollen hydrogel particles
were filled into a 5 mL graduated pipette with the minimum
scale of 50 mL. Experiments found that if this hydrogel particle
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loaded pipette was directly applied to 10 mM Cu solution, it
took about 2 h to achieve the steady volume readout. This time
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consuming response time is attributed to the diffusion of Cu
from sample solution into the 3D network of hydrogel particles
throughout the pipette. To shorten the response time for each
detection, about 80 diffusion holes (0.8 mm in diameter) were
uniformly engraved in the pipette wall with a laser beam (Fig. S3,
ESI†). In order to examine the effect of the particle size on sensitivity,
three size range particles of 40–60, 60–80, and 80–120 mesh (dried
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hydrogel) were applied to 1.0 mM Cu standard. Experimental
results displayed in Fig. S4 (see ESI†) indicate that the 80–120 mesh
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particles showed the best volume response to Cu . Further
reduction of hydrogel particle size requires the size reduction of
diffusion holes to prevent the leakage of particles from the
pipette. Restricted by the capability of the available laser instru-
ment, 80–120 mesh particles were chosen to construct the sensor. and (b) presence of 0.1 mM Cu
Fig. 1 TEM microphotographs of the freeze-dried less cross-linked
hydrogel (A) in the absence and (a) in the presence of 1.0 mM Cu ; and
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SEM microphotographs of the freeze-dried hydrogel (B) in the absence
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.
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By monitoring the volume variation along with response sensor exhibits the visually measurable volume responses pro-
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time, kinetics of Cu responsive shrinkage was studied, as is portional to Cu concentration in a wide range. That is, the
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shown in Fig. S5 (see ESI†). Graduated pipettes (5.0 mL) loaded sensor is potentially able to detect Cu without the requirement
with 4.5 mL of hydrogel particles were immersed in 1.0, 5.0, and for any external electrical device or power source. Fig. 3 offers the
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2.0 mM Cu solutions, respectively; their volume responses panorama of the sensor’s response curve to Cu ranging from
were recorded every 2 min. It was found that gradual shrinkage 0.1 to 500 mM. By a close observation of the trend of sensor
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of the hydrogel over elapsing time came to an end in 10 min for response with Cu concentration increase, a leveling off point
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all three concentrations. As it is, the sensor can respond rapidly corresponding to about 160 mM Cu is found. As demonstrated
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to Cu . It is noticed that the 0.51 mL of volume response, in S3 (see ESI†), the ratio of amine to carboxylic groups in hydrogel is
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14a,b
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corresponding to 1.0 mM Cu , is significant enough for quanti- about 4 : 1. According to the literature,
it is assumed that a Cu
tative readout. This result suggests the potential of the sensor for ion chelates four amine and one carboxylic groups to form a highly
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quantitative detection of Cu at mM levels. In comparison of our stable structure as shown in Fig. S6 (see ESI†). The range over which
1
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hydrogel with the reported hydrogel, both of them exhibit the the signal response linearly behaved with analyte concentration
sensitive response toward copper ions in spite of their difference is important for a sensor. In our case, the responsive volume
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in hydrogel composition, sensing principle, response time, varies linearly with Cu concentration over the range 0.1–0.8 mM,
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and the signal for quantification. While the reported hydrogel 0.8–12 mM, and 12–120 mM (Fig. 3 inset), respectively. 0.8 mM Cu is
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performed the observable proportional volume response to Cu
noticed to produce about 0.47 mL of volume response; a Cu
concentrations, our design is specified to approach the precise solution lower than this concentration may cause a volume
visual readout of volume variations for quantitative detection. In readout error exceeding 5%. Therefore, 0.8 mM is set as the
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addition, by removing the accumulated Cu with appropriate initial concentration for quantitative detection, and the first
chelating reagents, our hydrogel is expected to be recoverable linear range 0.1–0.8 mM permits semi-quantitative detection
rather than disposable.
while the other two for quantitative detection. To examine
In order to evaluate the selectivity of our volumetric sensor the sensor reproducibility, replicate detections with respect to
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toward Cu when it is used for water samples, other metals 0.8, 6.0 and 12 mM Cu were carried out. As we can see in
that possibly coexist in natural water were investigated in place Fig. S7 (see ESI†), the sensor shows less than 5% relative
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of 5.0 and 10 mM Cu with 1 and 10 fold higher concentrations standard deviations for all three cases. Such reproducibility is
under identical conditions, as is shown in Fig. 2. In comparison normally acceptable for quantitative detections. According to
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À1
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with the clear volume response to Cu , other tested metals the 1.3 mg L limit for Cu in drinking water issued by the US
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3+
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3+
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3+
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including Ag , Al , Ca , Cd , Co , Cr , Fe , Fe , Hg , Environmental Protection Agency, our sensor is capable for
Mg , Mn , Ni and Zn barely cause slight or negligible quantitative detection of Cu in drinking waters. To examine
volume responses. Such selectivity toward Cu matches that the sensor capability for real samples, Cu in swimming
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1
5
of reported selective chelation-based sensors. The developed pool water and tap water was determined. As summarized in
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sensor enables the differentiation of Cu from other common Tables S1 and S2 (see ESI†), the agreements with the ICP-MS
metal ions in natural waters with naked-eye observation. method and the recovery test results for spiking samples
In addition to the investigation of the selectivity of the proposed provide evidence for the accuracy and feasibility of our sensor
visual volumetric sensor, its capability for quantitative detection for detection of copper ions in water.
2
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of Cu was evaluated. As aforementioned in Scheme 1B, our
Fig. 2 Selectivity of the volumetric sensor. The heights of the vertical bars
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2+
represent the volume responses corresponding to 10 mM Cu and other 13
metal ions at a 10 fold concentration, respectively. The inset shows a photo-
graph of the sensor for the same investigation, with the difference that 5.0 mM
Fig. 3 Sensor calibration curve response to Cu concentrations from 0.1
to 500 mM. Insets of A, B, and C are the linear plots of the volume response
vs. Cu concentration in the range of 0.1 to 0.8 mM, 0.8 to 12 mM, and 12 to
120 mM, respectively.
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Cu and 5.0 mM other metal ions were used; red B stands for the blank.
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7
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8
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2
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Foundation of China (21175090).
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1
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