Ultrasensitive microchip based on smart microgel forreal-time online detection of trace threat analytesShuo Lin (林硕)a, Wei Wang (汪伟)a,1, Xiao-Jie Ju (巨晓洁)a,b, Rui Xie (谢锐)a, Zhuang Liu (刘壮)a, Hai-Rong Yu (余海溶)a,Chuan Zhang (张川)a, and Liang-Yin Chu (褚良银)a,b,c,1

aSchool of Chemical Engineering, Sichuan University, Chengdu, Sichuan 610065, China; bState Key Laboratory of Polymer Materials Engineering, SichuanUniversity, Chengdu, Sichuan 610065, China; and cCollaborative Innovation Center for Biomaterials Science and Technology, Sichuan University, Chengdu,Sichuan 610065, China

Edited by David A. Weitz, Harvard University, Cambridge, MA, and approved January 5, 2016 (received for review September 16, 2015)

Real-time online detection of trace threat analytes is critical forglobal sustainability, whereas the key challenge is how to efficientlyconvert and amplify analyte signals into simple readouts. Here wereport an ultrasensitive microfluidic platform incorporated withsmart microgel for real-time online detection of trace threat analytes.The microgel can swell responding to specific stimulus in flowingsolution, resulting in efficient conversion of the stimulus signalinto significantly amplified signal of flow-rate change; thus highlysensitive, fast, and selective detection can be achieved. We demon-strate this by incorporating ion-recognizable microgel for detectingtrace Pb2+, and connecting our platform with pipelines of tap waterand wastewater for real-time online Pb2+ detection to achieve timelypollution warning and terminating. This work provides a generaliz-able platform for incorporating myriad stimuli-responsive microgelsto achieve ever-better performance for real-time online detection ofvarious trace threat molecules, and may expand the scope of appli-cations of detection techniques.

microchip | microfluidics | smart microgel | ultrasensitive detection |real-time online detection

Timely detection of trace threat analytes that are harmful toenvironment and human health is critical for environmental

protection (1), disease treatment (2, 3), and epidemic prevention(4). The key challenge is how to efficiently convert and amplify theanalyte signal into simple readouts for real-time detection. Basedon the stimuli-responsive volume changes of smart hydrogels (5, 6),current techniques allow converting the stimulus signals into elec-trical or optical signals for detection. Generally, the volume changeof smart hydrogels can be converted to electric current throughfield-effect transistors and pressure sensors for detecting glucose(2.8 × 10−3 M) (7) and metal ions (8). These methods inefficientlyuse the 3D hydrogel deformation, thus possessing poor detectionlimit. Improved sensitivity can be achieved by converting thetarget signals into optical signals. Typically, upon volume change,photonic crystal hydrogels can change their lattice constantsto shift diffraction peaks for detecting trace analytes such asPb2+ (∼10−9 M) (9, 10), DNA (10−9 M) (11), and 3-pyridinecarbox-amid (12) via spectrometer, whereas hydrogel diffraction gratingscan adjust their refractive indexes to change the diffraction effi-ciency for detecting Ig-G (∼6 × 10−6 M) (13) and glucose (∼2.3 ×10−5 M) (14) via silicon photodiodes, resistors, and preampmodule. Alternatively, in response to target signals, hydrogelmicrocantilever can slightly bend to deflect laser light for detectingPb2+ (10−7 M) (15) and CrO4

2− (10−10 M) (16) via atomic force mi-croscopy, whereas fluorescently modified hydrogels can changetheir fluorescent intensity for detecting glucose (17), Hg2+ (10−8 M),and Pb2+ (10−9 M) (18) via fluorescence spectrophotometers.However, all these techniques require sophisticated equipmentand professionals for detection and analysis. To sum up, theplatforms with electrical signals provide easy use and low cost,but the detection limit is poor, whereas the platforms with opti-cal signals offer improved sensitivity, but require sophisticatedanalyzing protocols, which restrict the applications for real-time

online detection. Up to now, development of simple and ultra-sensitive detection platforms for real-time online detection oftrace analytes has remained a challenge.Herein, we report an ultrasensitive microchip based on stimuli-

responsive smart microgel for real-time detection of trace threatanalytes. The key unit of our detection platform is a microfluidicchip with glass-capillary microchannel integrated with cylinder-shaped smart microgel, which allows highly sensitive, fast, andselective detection of trace threat analytes. We demonstrate this byusing poly(N-isopropylacrylamide-co-benzo-18-crown-6-acrylamide)[P(NIPAM-co-B18C6Am)] microgel with NIPAM units as actua-tors and B18C6Am units as ion signal sensing receptors to selec-tively recognize trace Pb2+ (Fig. 1). As illustrated in Fig. 1A, themicrogel is initially shrinking in water at operation temperature(To) (Fig. 1B), which is above the volume phase transition tem-perature (VPTT) of the microgel in pure water (VPTT1). Whentrace Pb2+ appears, the B18C6Am units capture Pb2+ and formstable B18C6Am/Pb2+ host–guest complexes via molecular rec-ognition (6, 19) (Fig. 1 B and C). This leads to a VPTT shiftfrom VPTT1 to a higher VPTT2 due to the electrostatic repulsionamong the charged B18C6Am/Pb2+ complex groups (19–21). Thus,the microgel isothermally swells at To, due to the shift of VPTT to ahigher value than To and the enhanced osmotic pressure within themicrogel based on Donnan potential (19–21). By fabricating cylinder-shaped P(NIPAM-co-B18C6Am) microgel inside a capillary micro-channel, the interstice between the microgel and capillary creates acrescent-moon-shaped microspace for flowing fluids (Fig. 1D and E).

Significance

Real-time detection of trace threat analytes is critical for global sus-tainability. The key challenge is how to efficiently convert and am-plify the analyte signal into simple readouts. Here we report anultrasensitive microfluidic platform incorporated with stimuli-responsive smart microgel for real-time detection of trace threatanalytes. The microgel swells in response to specific analyte, thusconverting trace analyte concentration into significantly amplifiedsignal of flow rate change for highly sensitive, fast, and selectivedetection, which can be monitored on cell phone for timely warningand terminating of pollution. This work provides a generalizableplatform for incorporating myriad microgels to achieve ever-betterperformance for real-time detection of various trace threatmolecules,and may expand the scope of applications of detection techniques.

Author contributions: S.L., W.W., and L.-Y.C. designed research; S.L. performed research;S.L., W.W., X.-J.J., R.X., Z.L., H.-R.Y., C.Z., and L.-Y.C. analyzed data; and S.L., W.W., andL.-Y.C. wrote the paper.

The authors declare no conflict of interest.

This article is a PNAS Direct Submission.

Freely available online through the PNAS open access option.1To whom correspondence may be addressed. Email: wangwei512@scu.edu.cn or chuly@scu.edu.cn.

This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1518442113/-/DCSupplemental.

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Upon recognizing Pb2+, the microgel isothermally swells to a certaindegree depending on the Pb2+ concentration ([Pb2+]) (19, 21); as aresult, the flowing area of the crescent-moon-shaped microspacedecreases, and thus the flow rate drops correspondingly (Q → Q′, inwhich Q > Q′). According to the Hagen–Poiseuille law, the flow ratethrough a microchannel is governed by the fourth power of the hy-draulic equivalent diameter of flowing space (22). Thus, thePb2+-induced swelling of the P(NIPAM-co-B18C6Am) microgel incapillary microchannel greatly affects the flow rate. Therefore, with theproposed microchip, the trace Pb2+ signals can be efficiently con-verted into significantly amplified signals of flow rate change. Then,measurement of flow rates downstream the microchip via a simpleonline flowmeter (Fig. 1 F–H) allows quantitative detection oftrace Pb2+. Because the characteristic time of gel swelling is pro-portional to the square of a linear dimension of the hydrogel (23),the micrometer-scale size of microgel enables its rapid swellingupon recognizing Pb2+. Furthermore, the flowing of solution around

the microgel enables enhanced Pb2+ transfer into the microgelnetworks, which is also beneficial to the rapid swelling of microgel.As a result, ultrasensitive and real-time detection of Pb2+ can beachieved with the proposed platform.

Results and DiscussionFabrication of Microfluidic Chip Integrated with Cylinder-ShapedMicrogel. To fabricate the microfluidic chip integrated with cylinder-shaped microgel, an advanced rotation-based method is developedfor 360° uniform UV irradiation of the mask-covered capillary(Fig. 2 A and B), which is filled with aqueous solution containingmonomers B18C6Am and NIPAM, cross-linker N,N′-methylene-bis-acrylamide, and photoinitiator 2,2′-azobis(2-amidi-nopropanedihydrochloride). This approach efficiently converts the loadedmonomer solution (Fig. 2 C1 and D1) into uniform cylinder-shaped microgel (Fig. 2 C2 and D2, SI Appendix, Fig. S1, andMovie S1). Then, a coaxially placed cylinder bar of stainless steel is

Fig. 1. Schematics of Pb2+-detection platform equippedwith microchip incorporating P(NIPAM-co-B18C6Am)microgel. (A–C) P(NIPAM-co-B18C6Am) hydrogel canisothermally swell after recognizing Pb2+ via formingstable host–guest complexes. (D and E) By incorpo-rating cylinder-shaped P(NIPAM-co-B18C6Am) micro-gel inside the capillary as Pb2+ sensor, the trace Pb2+

signal can be efficiently converted into significantlyamplified signal of flow rate change (Q → Q′, in whichQ>Q′). (F–H) Pb2+-detection platform (F ), equippedwith a microfluidic chip for Pb2+ sensing based on theabove-mentioned principle and an online flowmeter forflow-rate measuring based on the flow-rate–dependenttemperature distribution (G and H), enables real-timeonline quantitative detection of trace Pb2+.

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skillfully introduced to stably support the microgel inside the cap-illary microchannel (Fig. 2 C3 and D3 and SI Appendix, Fig. S2).After removing the unpolymerized solution from the microchannel(Fig. 2 C4 and D4), microgel-based Pb2+ sensor is obtained. Finally,the Pb2+ sensor is fixed on a glass plate and simply connected toan online flowmeter by polyethylene tubes for constructing thePb2+-detection platform (SI Appendix, Fig. S3). The relationshipbetween the swelling rate and microgel size has been investigated bytransferring the microgel from pure water to 10−5 M Pb2+ solution.The ratio of the microgel volume at time t s to that at 0 s after thetransfer (NV) decreases with increasing the microgel radius at afixed time, indicating a faster swelling rate of smaller microgel (Fig.2E). The experimental data of the characteristic time for hydrogelswelling (τv), defined as the time at which the hydrogel reaches 99%of its equilibrated swelling volume, coincide well with the calculatedvalues obtained by the equation τv = r2/(π2 × D) (23) (Fig. 2F), inwhich D is the collective diffusion coefficient determined by thekinetic experiment, and r is the microgel radius.

Sensitivity of Detection Platform. Our detection platform exhibitshighly sensitive and fast detection performance. First, we de-termine that the optimal operation temperature of our platformis 34 °C, because the sensor shows the most significant equili-brated change of flow rate (ΔJ = Q – Q′) in response to each[Pb2+] at 34 °C compared with the flow rate of pure water (Q)(Fig. 3A). It is worth noting that, at 30 °C, the equilibrated

swelling size of microgel is still larger than the microchanneldimension; thus, the microgel volume is constrained by the capillarywall for blocking the microchannel. So, there is no flow in themicrochannel and the ΔJ remains nearly unchanged at 30 °C. At34 °C, significant decrease of flow rate can be detected after rec-ognizing Pb2+ with [Pb2+] varied from 10−10 M to 10−5 M within5 min (Fig. 3B) (see SI Appendix, Fig. S4 for detailed flow ratedata within the first 250 s). Especially, even in the [Pb2+] range from10−10 M to 10−8 M, which is much lower than the guideline value ofthe World Health Organization for drinking water (4.83 × 10−8 M),obvious decrease in flow rate can be detected, indicating the ultra-sensitivity of our platform. Meanwhile, the slope of ΔJt/ΔJmax curves(more details see SI Appendix, Fig. S5) at ΔJt/ΔJmax = 50% (S50)increases with increasing [Pb2+], where ΔJt is the ΔJ at time t s andΔJmax is the maximum value of ΔJ (Fig. 3C). Moreover, the timesrequired for change of ΔJt/ΔJmax from 5% to 50% (t50–t5) and forchange of ΔJt/ΔJmax from 5% to 90% (t90–t5) both decrease withincreasing [Pb2+] (Fig. 3C, Inset). All of the results indicate a fasterdynamic swelling rate at higher [Pb2+] due to the faster formation ofB18C6Am/Pb2+ complex groups. For accurate detection of [Pb2+],the quantitative relationship between Pb2+ concentration and flowrate change is obtained from Fig. 3D, as [Pb2+]=3 × 10−14 × (ΔJ)4.3.

Selectivity and Repeatability of Detection Platform. Our detectionplatform also shows excellent selectivity and repeatability for Pb2+

detection. Interferences from other ions in flow rate decrease only

Fig. 2. Fabrication of microgel-based Pb2+ sensor.(A and B) An advanced rotation-based method isdeveloped for fabricating uniform cylinder-shapedmicrogel in capillary microchannel, by insertingmonomer-solution–containing capillary fixed on amotor into two steel tubes (A) that are covered by amask to form an exposed part between tubes, andthen using rotation-assisted 360° uniform UV irradi-ation to achieve polymerization (B). (C and D) Sche-matics (C) and optical micrographs (D) showing thefabrication and fixation of microgel. In the monomer-solution–containing microchannel (C1 and D1), uni-form cylinder-shaped microgel is synthesized by rota-tion-assisted UV-polymerization (C2 and D2), and thenstably supported by skillfully introducing a coaxiallyplaced stainless steel bar (C3 and D3), followed withremoving the unpolymerized solution (C4 and D4).(E ) Effect of the microgel radius on the dynamicswelling ratio (NV = Vt/V0) of the cylinder-shapedmicrogel, where V0 and Vt are, respectively, themicrogel volume at 0 s and t s after transferring frompure water to 10−5 M Pb2+ solution. (F) Characteristictime for the swelling of microgels (τv) with differentradius.

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occur when the ion concentrations are larger than 10−6 M (Fig.4A). Under concentration of 10−6 M, Pb2+ results in a significantdecrease of flow rate (60 μL/min), while interferences fromother ions are all negligible (less than 2 μL/min) (Fig. 4B). Suchinterferences from Ba2+, Sr2+, K+, and Na+ are negligible evenwhen increasing their concentrations 100× that of [Pb2+] at[Pb2+] = 10−6 M, or even 1,000× that of [Pb2+] at [Pb2+] = 10−5 M(Fig. 4C), indicating excellent selectivity. Moreover, the detec-tion platform can be repeatedly used by simply and alternativelywashing off the captured Pb2+ with water at 55 °C and 25 °C (SIAppendix, Fig. S6). In water at 55 °C (>VPTT2), the microgelshrinks and makes the B18C6Am units close to each other,producing electrostatic repulsions among the ions against theformation of stable B18C6Am/Pb2+ complexes, and leading todecomplexation of Pb2+ from B18C6Am units (20). Meanwhile,the decrease of inclusion constant upon heating also facilitates thePb2+ decomplexation (21). In water at 25 °C, the microgel swellsagain and takes fresh water inside for Pb2+ removal. Repeat of

such a shrinking/swelling cycle upon heating and cooling enhancesthe water transporting into and out of the microgel network forremoving Pb2+. The detection platform after different wash cyclesis used for Pb2+ detection to estimate the recovery of the detectingperformance. For detection platforms used for detecting different[Pb2+], each of the flux recovery ratios [RF = (ΔJmax – ΔJt)/ΔJmax]can reach 100% after different wash cycles (Fig. 4D). The cycletimes required for 100% recovery increase with increasing [Pb2+](Fig. 4D and SI Appendix, Fig. S7A). The detection platforms showexcellent recovery performance for repeated detection with highaccuracy (SI Appendix, Fig. S7B). The detection mechanism andportability of our detection platform enable its flexible and facileutilization as an online unit for real-time detection of Pb2+. This isdemonstrated by using the platform for real-time online detectionof Pb2+ in tap water and in wastewater from a model industrialfactory for pollution warning and terminating via cell-phone mon-itoring (SI Appendix, Figs. S8–S10 and Movies S2 and S3).

Fig. 3. Highly sensitive and fast detection of Pb2+. (A) Effect of temperature and [Pb2+] on the equilibrated change of flow rate (ΔJ = Q – Q′) after switching purewater to Pb2+ solution for 15min. (B) Time-dependent flow-rate changes in response to different [Pb2+] values at 34 °C. (C) Effect of [Pb2+] on the slope (S50) ofΔJt/ΔJmax curves at ΔJt/ΔJmax = 50% in SI Appendix, Fig. S5. (Inset) Effect of [Pb2+] on the time required for change of ΔJt/ΔJmax from 5% to 50% (t50–t5) and forchange of ΔJt/ΔJmax from 5% to 90% (t90–t5). (D) Quantitative relationship between [Pb2+] and ΔJ after switching pure water to Pb2+ solution for 15 min.

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ConclusionsWe have demonstrated the real-time detection of Pb2+ by developingultrasensitive microchips incorporating P(NIPAM-co-B18C6Am)smart microgel. The proposed detection platform exhibits highlysensitive, fast, and selective detection performance, and pos-sesses flexible and facile utility as an online unit for real-timePb2+-detection. Especially, the measured [Pb2+] value can be con-veniently displayed on the popular cell phone, with which timelywarning or even terminating of Pb2+ pollution can be easilyachieved by presetting a critical level with an APP software. Such acombination of highly sensitive and selective detection, real-timeonline operation, and simple readouts, along with the easy con-struction, makes the proposed microchip platforms ideal candi-dates for further investigations and applications. The strategy ofthe ultrasensitive microchip integrated with smart microgel pre-sented here circumvents the difficulties in simultaneously reducing

the detection limit and improving the easy-to-operate property ofdetection techniques for trace analytes. It can be used to constructversatile new detection platforms for real-time detection of variouskinds of trace analyte signals just by incorporating other stimuli-responsive microgels (6, 24–28), which might be a fertile areaof research. Due to the excellent ultrasensitive, fast, and easy-to-operate properties, the detection platforms equipped with micro-chips incorporating smart microgel will provide ever-better perfor-mances in myriad applications including environmental protection,disease diagnosis and epidemic prevention, and may open up newareas of application for hydrogel-based detection techniques.

Materials and MethodsIn Situ Synthesis of Cylinder-Shaped Microgel Within Glass Capillary. The uniformcylinder-shaped poly(N-isopropylacrylamide-co-benzo-18-crown-6-acrylamide)[P(NIPAM-co-B18C6Am)] microgel is in situ synthesized within a glass capillary

Fig. 4. Highly selective and excellent repeatability of Pb2+-detection platform. (A) Equilibrated flow rates at different concentrations of Na+, Sr2+, K2+, Ba2+, andPb2+ at 34 °C. (B) Effect of ion species on the equilibrated change of flow rate ΔJ after switching pure water to the solution containing each ion (10−6 M) for 15 minat 34 °C. (C) Effects of interfering ions on the flow rate changes (ΔJ). The interfering ions include Ba2+, Sr2+, K+, and Na+. (D) Dynamic flux recovery ratio (RF) ofdetection platforms after detecting different [Pb2+] concentrations and washing with pure water in a wash cycle manner shown in SI Appendix, Fig. S6.

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microchannel by developing an advanced rotation-based UV-irradiationmethod. For synthesis of the cylinder-shaped microgel, typically, 2 mL aqueoussolution containing 0.3 mmol monomer benzo-18-crown-6-acrylamide (B18C6Am)(TCI) and 2.0 mmol monomer N-isopropylacrylamide (NIPAM) (TCI), 0.04 mmolcross-linkerN,N′-methylene-bis-acrylamide (Chengdu Kelong Chemicals) and 0.037mmol photoinitiator 2,2′-azobis(2-amidi-nopropane dihydrochloride) (TCI) is in-jected into a glass capillary, with inner diameter of 250 μm and outer diameter of960 μm. The mole ratio of crown ether to NIPAM is set at 3:20 in our work, be-cause the as-prepared microgel shows good response performance with the moleratio of crown ether to NIPAM at 3:20, and further increase of the mole ratioshows little effect in improving the response performance (19). The monomer-solution-loaded capillary, with one end fixed on a rotating motor, is inserted intotwo stainless steel tubes with inner diameter of 1,000 μm. The stainless steel tubesare placed on a hot and cold stage (mk1000, Instec) to keep the synthesis tem-perature at 0 °C. The exposed part of capillary between the two steel tubes iscovered by a patterned mask, with a transparent rectangular area (size: 130 μm ×1 cm) for UV irradiation. Then, the monomer-solution-loaded capillary is rotatedat a speed of 60 rpm and irradiated with UV light (λ = 365 nm) for 120 s to in situsynthesize a uniform cylinder-shaped microgel inside the capillary (Movie S1).Compared with the traditional one-direction UV-irradiation method that usuallyleads to nonuniform cylinder-shaped microgel (SI Appendix, Fig. S1 A and B), sucha rotation-based UV-irradiation method ensures 360° uniform UV irradiation forfabricating microgel with uniform cylinder shape (SI Appendix, Fig. S1 C and D).

Construction of Pb2+ Detection Platform. After the UV-initiated synthesis of themicrogel inside capillary, the microgel-incorporated capillary is used for assemblyof the Pb2+ detection microfluidic chip (SI Appendix, Fig. S2). First, the microgel-incorporated capillary is fixed on a glass slide by epoxy resin, and clamped by twofixtures for fixation (SI Appendix, Fig. S2 A and B). Then, another capillary, withinner diameter of 170 μm and outer diameter of 960 μm, is placed into the in-terstice between the two fixtures and fixed on the glass slide for coaxial align-ment with the first capillary (SI Appendix, Fig. S2C). After that, a cylinder bar ofstainless steel with diameter of 165 μm is coaxially inserted into the capillaries tosupport the microgel, followed with fixation of the cylinder bar on the glass slideand seal of the second capillary by epoxy resin, and finally removal of the fixtures(SI Appendix, Fig. S2 D and E). The coaxiality of the cylinder bar, glass capillary,and cylinder-shaped microgel is important for the microgel fixation, becauseuncoaxial alignment of the microgel and cylinder bar leads to the lean of themicrogel under flow impact. After the assembly, the unpolymerized solution inthe capillary is removed, and thus the microgel-based Pb2+ sensor is obtained.Next, two needles are, respectively, connected to the two ends of the microgel-incorporated capillary, and sealed with epoxy resin as the inlet and outlet forfabricating the microfluidic detection chip (SI Appendix, Fig. S3A). Finally, theoutlet of the detection chip is connected to a flowmeter of microfluidic controlsystem (FLU_L, Fluigent) by polyethylene pipes for constructing the Pb2+ detectionplatform (SI Appendix, Fig. S3B). The dynamic swelling behaviors of the cylinder-shaped microgels with different sizes are studied by transferring the microgel

from pure water to 10−5 M Pb2+ solution and measuring its dynamic size changeusing optical microscope (BX61, Olympus). The calculated values of τv for themicrogels with different sizes are obtained from equation τv = r2/(π2×D) (23),based on D derived from the dynamic swelling behaviors.

Determination of the Optimal Operation Temperature for the DetectionPlatform. To determine the optimal operation temperature for the Pb2+

detection, effects of temperature and Pb2+ concentration ([Pb2+]) on theflow rates are studied. First, pure water is supplied into the detection plat-form at a certain temperature. Then, Pb2+ solution with a certain concen-tration is supplied into the detection platform at the same temperature asthat of pure water. Both pure water and Pb2+ solution are supplied under aconstant pressure of 30 kPa by the microfluidic control system. The equili-brated flow rates of pure water and Pb2+ solution after being supplied for15 min are measured by the flowmeter for evaluating the equilibrated flowrate changes (ΔJ). During the experiments, the temperature of pure waterand Pb2+ solution is varied from 30 °C to 40 °C, and [Pb2+] is varied from 10−9 Mto 10−4 M. The temperature at which the detection platform shows the mostsignificant ΔJ value is defined as the optimal operation temperature.

Test of the Response Time and Sensitivity of the Detection Platform. The re-sponse time and sensitivity of the detection platform are investigated bymonitoring the dynamic change of flow rates after switching pure water toPb2+ solutions with different concentrations from 10−10 M to 10−2 M.

Test of the Selectivity of the Detection Platform. The selectivity of the de-tection platform is first investigated by monitoring the dynamic change offlow rates after switching pure water to Pb2+ solution as well as solutionscontaining other interfering ions with different concentrations from 10−10 Mto 10−3 M. Then, the equilibrated flow rate changes (ΔJ) after switching purewater to solution that concurrently contains Pb2+ and interfering ions for15 min are studied. The interfering ions include Ba2+, Sr2+, K+, and Na+, eachwith concentration 1∼1,000× as large as the [Pb2+]. All of the solutions areflowed at 34 °C under a constant pressure of 30 kPa.

Test of the Repeatability of Detection Platform. The repeatability of detectionplatform for Pb2+ detection is investigated after alternatively and repeatedlyinjecting pure water at 55 °C and 25 °C into the detection platform, each for3 min, for Pb2+ removal. The Pb2+ removal performance after different washcycles is estimated by measuring the flux recovery ratio (RF). After complete Pb2+

removal (RF = 100%), the detection platform is used for repeated Pb2+ detection(SI Appendix, Fig. S7).

ACKNOWLEDGMENTS. The authors gratefully acknowledge support fromthe National Natural Science Foundation of China (21136006, 21490582) andState Key Laboratory of Polymer Materials Engineering (sklpme2014-1-01).

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