A Novel Membrane Detector Based on Smart Nanogels for

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A Novel Membrane Detector Based on Smart Nanogels for Ultrasensitive Detection of Trace Threat Substances Pei-Jie Yan, Fan He, Wei Wang, Shi-Yuan Zhang, Lei Zhang, Ming Li, Zhuang Liu, Xiao-Jie Ju, Rui Xie, and Liang-Yin Chu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b12615 • Publication Date (Web): 27 Sep 2018 Downloaded from http://pubs.acs.org on September 28, 2018

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ACS Applied Materials & Interfaces

A Novel Membrane Detector Based on Smart Nanogels for Ultrasensitive Detection of Trace Threat Substances Pei-Jie Yan,†,§ Fan He,†,§ Wei Wang,*,†,‡ Shi-Yuan Zhang,† Lei Zhang,† Ming Li,† Zhuang Liu,†,‡ Xiao-Jie Ju,†,‡ Rui Xie,†,‡ and Liang-Yin Chu*,†,‡

†

School of Chemical Engineering, Sichuan University, Chengdu, Sichuan 610065, P.

R. China ‡

State Key Laboratory of Polymer Materials Engineering, Sichuan University,

Chengdu, Sichuan 610065, P. R. China

KEYWORDS:

crown ether; host−guest complexation; membrane detectors; smart

nanogels; ultrasensitive detection

1 ACS Paragon Plus Environment

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ABSTRACT:

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A novel membrane detector is developed by a facile strategy

combining commercialized membrane and smart nanogels, for ultrasensitive and highly-selective real-time detection of trace threat substances.

Based on nanogel

filtration and polydopamine adhesion, the membrane detector is fabricated by simply immobilizing smart nanogels onto the multiple pores of a commercialized membrane as the nanosensors and nanovalves. Pb2+-responsive

This is demonstrated by incorporating

poly(N-isopropylacrylamide-co-acryloylamidobenzo-18-crown-6)

nanogels in the straight pores of a commercialized polycarbonate membrane for ultrasensitive and highly-selective real-time detection of trace Pb2+.

When

selectively recognizing the Pb2+ in solution, the smart nanogels in the membrane pores swell, which lead to trans-membrane flux change.

Quantitative detection of

Pb2+ concentration can be achieved by simply measuring the flow rate of the trans-membrane flow. Due to the multiple nanochannels of nanogel-immobilized pores in the membrane for Pb2+-sensing and flux regulating, ultrasensitive and highly-selective real-time detection of trace Pb2+ with concentration as low as 10-10 mol L-1 can be achieved.

The nanogel-immobilized membrane detector offers a

flexible platform to create versatile new membrane detectors by incorporating diverse smart nanogels for ultrasensitive and highly-selective real-time detection of different trace threat substances.


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INTRODUCTION Detection of trace chemical/biological substances, which are harmful to human health even with ultralow concentration, is critical for various applications such as diagnosis of diseases1,2 and protection of environment.3,4

For example, due to the easy

accumulation in human body, Pb2+ with even trace amount can cause renal and neurological damages as well as brain disorders, especially for children.

Thus,

required by the World Health Organization (WHO), Pb2+ concentration ([Pb2+]) in drinking water must be less than 4.83 × 10-8 mol L-1.

Thus, real-time detection of

these trace threat substances with ultra-sensitivity and high-selectivity is highly required.

For such detection, the key challenge is the efficient conversion and

amplification of the signal of trace threat substances into simply detectable output. Traditional techniques such as inductively-coupled plasma spectroscopy,5-8 atomic absorption spectroscopy,9-12 electrochemical method,13-16 and fluorescent probes,17-19 can convert the signals of trace threat substances into electrical and optical signals for detection.

However, these techniques suffer from expensive detection apparatus,

time-consuming sample preparation, and troublesome operation and analysis process. Moreover, although electrochemical methods are easy and low-cost, electrodes modified with inorganic materials usually cannot achieve highly-selective detection of threat substances such as Pb2+, due to the severe interference from other metal ions;14 while those modified with enzymes usually suffer from high-cost and poor store ability.13,16 Compared with the above-mentioned techniques, responsive polymeric materials that can change their physical/chemical properties in response to specific



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stimuli,20-22 enable flexible conversion of the stimulus signals into diverse output signals.

Therefore, the responsive polymeric materials provide good flexibility in

signal conversion and amplification to develop advanced detection systems for detecting trace threat substances. Typically, several detection techniques have been developed based on responsive polymeric materials to convert the signal of trace threat substances into electrical, optical or flow-rate readouts for detection of such substances.

For electrical

detection, field-effect transistor based on smart hydrogel can convert glucose concentration signals into electrical signals, due to the glucose-responsive change of hydrogel volume and surface charge, for detecting glucose (~2.8×10-3 mol L-1).23 For optical detection, smart hydrogel gratings can change their diffraction efficiency upon responsive volume changes for detecting Pb2+ (~10-6 mol L-1),24 human immunoglobulin-G (~6×10-6 mol L-1),25 and glucose (~2.3×10-5 mol L-1).26 When incorporated with fluorescent molecules and photonic crystals, smart microgels and hydrogels can convert their responsive volume changes into fluorescent intensity change,27-29 and diffraction peak shift,30,31 for detecting Pb2+ (~10-9 mol L-1),32,33 glucose (~2.5×10-3 mol L-1)34, Hg2+ (~10-8 mol L-1),32,35 DNA (~10-9 mol L-1)36 and 3-pyridinecarboxamid.37

Alternatively, smart hydrogel microcantilevers can bend

when recognizing specific ions, and then change the position of laser deflection for sensitive detection of heavy metal ions such as Pb2+ (~10-6 mol L-1),38 and CrO42(~10-10 mol L-1).39

However, although these techniques enable sensitive detection of

different substances, they usually require troublesome process for device fabrication


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and sample preparation, and sophisticated analyzing protocols, which restrict their applications for real-time detection.

The detection systems based on flow rate

measurement allow easy connection of the detector with solution-flowing pipeline, which provide an efficient way for real-time online detection of trace threat substances.

For example, microchip with smart microgel valve allows

[Pb2+]-dependent volume change to adjust the flowing area in microchannel for flow-rate change.

The microchip can be incorporated with water pipeline for

sensitive and real-time online detection of Pb2+ (10-9 mol L-1).40

However, the

microchip fabrication still requires troublesome and skillful manual assembly.

By

grafting Pb2+-responsive polymers in the pores of commercialized membranes, simple detection systems can be constructed for adjusting the trans-membrane flux for detecting Pb2+ (10-6 mol L-1).41

However, although the membrane detection system

provides a simple strategy for Pb2+ detection, it still suffers from limited control of the plasma-grafting process for fabricating the grafting membrane, and limited sensitivity for detecting trace Pb2+ at the level required by WHO (4.83×10-8 mol L-1).

Therefore,

development of a facile and efficient strategy to fabricate simple detection system for ultrasensitive and highly-selective real-time detection of trace threat substances is still highly desired. Here, a novel membrane detector, based on simple integration of commercialized porous membrane with smart nanogels, is developed for ultrasensitive and highly-selective real-time detection of trace threat substances.

The straight pores of

the membrane provide multiple nanochannels for flowing fluids.


The smart


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nanogels, that can respond to specific stimulus, are anchored onto the pore nanochannels by simple filtration and polydopamine (PDA) adhesion (Figure 1a-c). This can create nanogel-immobilized membrane detection systems for ultrasensitive and highly-selective detection.

We demonstrate this by simply incorporating

Pb2+-responsive nanogels in the straight pores of commercialized polycarbonate (PC) membrane as nanosensors and nanovalves for ultrasensitive and highly-selective real-time

detection

of

trace

Pb2+.

The

nanogels

are

consisting

of

poly(N-isopropylacrylamide-co-acryloylamidobenzo-18-crown-6) (poly(NIPAM-co-AAB18C6)) (PNB) networks, with crown ether units for Pb2+ recognition and poly(N-isopropylacrylamide) (PNIPAM) backbone for actuation. When the crown ether groups recognize Pb2+ and form stable 18-crown-6/Pb2+ host-guest complexes, the volume phase transition temperature (VPTT1) shifts to a higher temperature (VPTT2) (Figure 1d).

When operated at an optimal detection

temperature (Td) between VPTT1 and VPTT2, the nanogels in the membrane pores can isothermally swell when [Pb2+] increases (Figure 1e,1f), leading to trans-membrane flux change for ultrasensitive [Pb2+] detection.

Since the nanogel-immobilized pores

of the membrane provide multiple nanochannels for Pb2+-sensing and flux regulating, ultrasensitive and highly-selective real-time detection of trace Pb2+ with concentration as low as 10-10 mol L-1 can be achieved.

The membrane detector based on

immobilized smart nanogels provides a versatile and advanced detection platform for ultrasensitive and highly-selective real-time detection of trace threat substances.


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EXPERIMENTAL SECTION Materials.

N-Isopropylacrylamide (NIPAM) (98%, TCI) was recrystallized using

a hexane/acetone mixture.

4’-Amino-benzo-18-crown-6 (AB18C6) was synthesized

from 4’-nitro-benzo-18-crown-6 (98%, TCI) based on our previous work.42 Ammonium persulfate, N,N’-methylene-bis-bisacrylamide, sodium dodecyl sulfate, acrylic acid (AAc) and tris(hydroxymethyl)aminomethane (Tris) were purchased from Chengdu Kelong Chemicals. hydrochloride Sigma-Aldrich.

(EDC)

and

1-(3-(Dimethylamino)propyl)-3-ethyl carbodiimide dopamine-hydrochloride

were

purchased

from

Deionized water (18.2 MΩ, 25 °C) from a Milli-Q Plus water

purification system (Millipore) was used throughout this work. Fabrication of PNB Nanogels.

PNB nanogels were prepared by first

synthesizing poly(N-isopropylacrylamide-co-acrylic acid) (P(NIPAM-co-AAc)) (PNA) nanogels (Figure S1), followed by AB18C6 modification (Figure S2).

First, for

preparing the PNA nanogels, monomers NIPAM (2.26g) and AAc (0.6171g), initiator ammonium persulfate (0.0651g), crosslinker N,N’-methylene-bis-acrylamide (0.044g) and surfactant sodium dodecyl sulfate (0.0329g) were dissolved in 250 mL deionized water.

The solution was then bubbled with nitrogen for 30 min to remove the

dissolved oxygen.

Next, the solution was heated to 70 oC and reacted for 4 h under

nitrogen atmosphere.

After the reaction, the solution was cooled down to room

temperature, followed with filtration and dialysis to obtain purified suspension of PNA nanogels.



Page 8 of 40

Second, PNB nanogels were prepared by modifying the PNA nanogels with AB18C6 via condensation reaction using EDC as dehydration catalysts, based on our previous work.42

Briefly, an aqueous solution containing PNA nanogels and

AB18C6 was mixed with another aqueous solution containing EDC, and then reacted at 4 oC under stirring and nitrogen atmosphere for 24 h.

After the reaction, the

solution was dialyzed against deionized water for 1 week to obtain suspension of purified PNB nanogels. A piece of commercialized

Preparation of Nanogel-Immobilized Membranes.

PC membrane (ATTP, Millipore) (Diameter = 47 mm) with pore diameters of 800 nm (PC-800) was used for immobilization of the PNB nanogels via filtration and PDA-based adhesion. Meanwhile, another piece of PC membrane, with same size, but with pore diameters of 220 nm (PC-220) was attached on the bottom of PC-800 membrane to hold up the nanogels (Figure 1a).

For nanogel immobilization, the

combined two pieces of PC membranes were fixed inside an ultrafiltration cup (MSC050, MOSU).

Aqueous suspension of PNB nanogels (2 mg mL-1) was filtrated

through the combined membranes with pressure of 0.1 MPa at 50 oC (Figure 1b). After that, the PC-800 membrane was detached from the PC-220 membrane and cooled down to 10 oC to let the PNB nanogels swell.

Then, the PC-800 membrane

was placed in Tris solution (10 mL, pH=8.5) containing dopamine (DA) (2 mg mL-1) and shaken for 2 h at 25 oC for immobilization of PNB nanogels (Figure 1c).

At last,

the nanogel-immobilized PC membrane was further washed with deionized water to remove un-immobilized PNB nanogels and unreacted DA.


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Construction of Membrane Detector for Pb2+-Detection.

To construct the

Pb2+-detection system, the nanogel-immobilized PC membrane was incorporated into a filter and then enveloped by a heating jacket for temperature control.

A

microfluidic control system (MFCS-FLEX 3C, Fluigent), coupled with a buffer tank, was used to inject the solution into the membrane detector at a constant pressure. The flow rate of flows that passed through the nanogel-immobilized membrane was measured by a flowmeter (FLU_L, Fluigent) coupled with a computer for data analysis (Figure 2). Chemical Composition and Morphological Characterization of PNB Nanogels and Nanogel-Immobilized Membranes.

The chemical composition of PNB

nanogels was characterized by Fourier transform infrared spectrometry (FT-IR) (IR Prestige-21, Shimadzu).

The sample for FT-IR characterization was freeze-dried and

tested by KBr disk technique.

The morphology of PNB nanogels in dried state was

observed by atomic force microscope (AFM) (MM8, BRUKER).

The sample for

AFM characterization was dried at room temperature. The morphologies of PC-800 membrane and nanogel-immobilized PC membrane were observed by scanning electron microscope (SEM) (TM3030, HITACHI).

The

membrane samples were treated with liquid nitrogen and then fractured mechanically for cross-section observation.

All the membrane samples for SEM observation were

sputter-coated with gold for 120 s. Study on the Pb2+-Responsive Properties of PNB Nanogels.

To investigate the

Pb2+-responsive properties of PNB nanogels at different temperatures, their



[Pb2+]-dependent diameter changes in aqueous solutions at temperatures ranging from 13 oC to 64 oC were measured by dynamic light scattering (DLS) (Zetasizer Nano ZS90, Malvern).

The highly diluted nanogels-containing dispersion was equilibrated

for 180 s at each predetermined temperature.

Similarly, the Pb2+-dependent diameter

changes of PDA-coated PNB nanogels at different temperatures were measured by DLS, to investigate the influence of dopamine on the Pb2+-responsive property of PNB nanogels.

For preparing the PDA-coated PNB nanogels, briefly, dried PNB

nanogels (0.003 g) were dispersed in Tris solution (3 mL) with pH = 8.5, followed with dissolution of DA (2 mg mL-1).

The PNB-nanogel-containing solution was

shaken for 2 h to modify the PNB nanogels with a PDA coating, and finally purified by centrifugation at 10000 rpm.

Moreover, to study the effect of DA on the

Pb2+-responsive property of PNB nanogels, absorption spectra of DA-containing solution, Pb2+-containing solution, and aqueous solutions containing both DA and Pb2+ were also tested by UV-vis spectrometer (UV-1700, Shimadzu), with a wavelength range from 200 nm to 350 nm.

The concentration of DA in the aqueous

solutions was 0.25 mg L-1, and the [Pb2+] was varied from 10-9 mol L-1 to 10-6 mol L-1. Study on the Optimal Detection Temperature and Sensitivity of Membrane Detector.

The optimal detection temperature (Td) of the membrane detector for

ultrasensitive Pb2+-detection was determined by studying the effects of temperature and [Pb2+] on the trans-membrane flux.

The temperature, at which the most

significant change of trans-membrane flux is obtained, is defined as the optimal detection temperature. Briefly, Pb2+-containing solution with a certain [Pb2+] was


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supplied into the membrane detector under a constant pressure of 0.015 MPa by the microfluidic control system.

The [Pb2+] in the Pb2+-containing solution was varied

from 10-10 mol L-1 to 10-6 mol L-1.

With a fixed [Pb2+], the flows were equilibrated

for 20 min at each predetermined temperature, and the equilibrated flow rate of flows that went through the membrane at different temperatures in the range from 30 oC to 50 oC was measured by the flowmeter. Study on the Repeatability and Selectivity of Membrane Detector.

To

investigate the performance of the membrane detector for highly-selective [Pb2+] detection, their trans-membrane flux in aqueous solutions containing interfering ions at optimal detection temperature was measured.

Each of the solutions contains one

type of different interfering ions, including K+, Na+, Ba2+, Ca2+, Mg2+, and Sr2+.

The

concentration of the interfering ion in each solution was 10-6 mol L-1. Since the Pb2+-detection was based the reversible volume phase transitions of PNB nanogels for flux control, to evaluate the repeated Pb2+-detection performance, the flow rate changes of membrane detector during repeated swelling/shrinking upon heating (50 oC) and cooling (10 oC) cycles were measured.

RESULTS AND DISCUSSION Choice of Materials. Among the materials usually used for specific detection of Pb2+ (Table S1), the 18-crown-6 units can provide crown ether cavities to selectively recognize specific metal ions and form stable host-guest complexes.

Due to the

largest complex constant between 18-crown-6 and Pb2+ as compared to other metal



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ions, here 18-crown-6 is employed as the sensor for Pb2+ recognition.

PNIPAM, a

thermo-responsive polymer that allows volume phase transition when changing its hydrophilic/hydrophobic equilibrium, is used as the actuator.

When combining

PNIPAM with 18-crown-6 by using AAc via first co-polymerization and then condensation reaction, PNB nanogels that can isothermally swell in response to Pb2+ can be obtained as the nanosensors and nanovalves.

Commercialized low-cost PC

membrane with regular straight pores, is used to provide multiple microchannels for incorporating the PNB nanogels.

Combination of the PC membrane and PNB

nanogels based on the adhesion property of PDA can simply produce the nanogel-immobilized membrane for detection of Pb2+. Preparation of Pb2+-Responsive PNB Nanogels.

Pb2+-responsive PNB nanogels

used as the Pb2+-responsive sensors in the membrane detector are synthesized by precipitation copolymerization of NIPAM and AAc, followed with AB18C6 modification via condensation reaction.

Figure 3a shows the FT-IR spectra of PNB

nanogels, with PNIPAM nanogels and PNA nanogels as the control groups.

For

PNIPAM nanogels, the peak at 1670 cm-1 is attributed to the carbonyl groups, while the ones at 1387 cm-1 and 1375 cm-1 are attributed to the isopropyl groups.

Both

peaks from the isopropyl groups can be observed for PNA nanogels and PNB nanogels, confirming the existence of PNIPAM backbones in PNA and PNB nanogels. For PNA nanogels, the peak at 1710 cm-1 is attributed to the carboxylic groups.

This

peak disappears after modification of PNA nanogels with AB18C6, indicating the conversion of the carboxylic groups via the condensation reaction.


Meanwhile, for

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PNB nanogels, the peaks at 1516 cm-1 and 1130 cm-1 are attributed to the C=C skeletal stretching vibration of phenyl ring and the C-O-C asymmetric stretching vibration of R-O-R', respectively.

The results confirm the successful incorporation

of 18-crown-6 groups in the PNA nanogels for preparing PNB nanogels.

Moreover,

as shown in the AFM images (Figure 3b), the PNB nanogels in dried state exhibit uniform spherical shapes, with an average diameter of 450 nm. Pb2+-Responsive Properties of PNB and PDA-Coated PNB Nanogels.

The

PNB nanogels enable volume phase transitions in response to [Pb2+] changes. Meanwhile, due to the thermo-responsive PNIPAM backbone, the PNB nanogels show different Pb2+-responsive volume phase transition behaviors at different temperatures.

As shown in Figure 4a, when placed in aqueous solution with

[Pb2+]=0 mol L-1, the diameter of PNB nanogels decreases from 831.4 nm to 330.9 nm, with increasing temperature from 13 oC to 64 oC.

Similar volume shrinking

behaviors can be obtained upon increasing temperature when placing the PNB nanogels in Pb2+-containing solutions with fixed [Pb2+].

Meanwhile, with a fixed

temperature, the diameter of PNB nanogels increases with increasing [Pb2+] from 0 mol L-1, 10-4 mol L-1, to 10-3 mol L-1, indicating the excellent Pb2+-responsive volume phase transition behavior.

Such Pb2+-induced volume changes of PNB nanogels

allow regulation of the pore size when they are incorporated into the membrane pores, for converting the [Pb2+] signal into detectable flux changes. To evaluate the Pb2+-induced volume changes of PNB nanogels at each temperature, Pb2+-induced swelling ratio (R1), defined by Equation (1), is used.



R1 =

DPb2+

Page 14 of 40

(1)

Dwater

where DPb 2+ and Dwater are respectively the diameters of PNB nanogels or PDA-coated PNB nanogels in Pb2+-containing solution and pure water at the same temperature.

As shown in Figure 4b, for PNB nanogels in Pb2+-containing solutions

with [Pb2+] of 10-3 mol L-1 and 10-4 mol L-1, both of the R1 values first increase with increasing temperature from 13 oC to 37 oC, and then decrease with increasing temperature from 37 oC to 64 oC.

The maximum R1 value at 37 oC means that, at 37

o

C, the PNB nanogels in water can achieve largest volume change when responding to

[Pb2+] changes. To decorate the PNB nanogels onto the membrane pore surface, DA is utilized as the binding agents for the nanogel immobilization.

The versatile adhesion property

of DA on diverse materials during self-polymerization,43-45 ensures the tight binding between PNB nanogels and the pore surface of PC membrane. the Pb2+-responsive property of PNB nanogels is investigated.

Effect of the DA on Firstly, the absorption

spectra of DA-containing solution, Pb2+-containing solutions and DA/Pb2+ mixed solutions are tested by UV-vis spectrometer.

As shown in Figure S3, there are

absorption peaks for DA-containing solution, but no absorption peaks appear for the Pb2+-containing solutions with different [Pb2+].

Meanwhile, for the DA/Pb2+ mixed

solutions with different [Pb2+], no additional absorption peaks appear as compared with the one of DA solution, indicating no influence of DA on the complexation between the crown ether and Pb2+.


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Figure 5a shows the excellent volume phase transitions of PDA-coated PNB nanogels in response to temperature and [Pb2+].

In pure water, with increasing

temperature from 13 oC to 64 oC, the diameter of PNB nanogels decreases from 634.2 nm to 343.2 nm.

As compared with PNB nanogels, due to the coating of the PDA

layer on the nanogel surface, the PDA-coated PNB nanogels show a slightly decreased diameter at each temperature.

Meanwhile, with fixed temperature, the

diameter of PDA-coated PNB nanogels increases when increasing [Pb2+] from 0 mol L-1, 10-4 mol L-1, to 10-3 mol L-1, indicating excellent Pb2+-responsive volume changes. Moreover, for PNB nanogels in solutions with [Pb2+] of 10-3 mol L-1 and 10-4 mol L-1, both of their R1 values first increase and then decrease with increasing temperature, showing a maximum value at 40 oC (Figure 5b).

As compared with that of the PNB

nanogels (Figure 4b), such an increase in the optimal detection temperature is resulted from the VPTT shift caused by the incorporation of hydrophilic PDA units in the crosslinked hydrogel networks.46

The excellent Pb2+-responsive volume phase

transition behaviors of the PDA-coated PNB nanogels ensure the good performance of the immobilized nanogels for regulating pore size for flux control. Construction of Nanogel-Immobilized Membrane Detector.

To construct the

nanogel-immobilized membrane detector, the PNB nanogels, used as Pb2+-sensors are incorporated into the commercialized PC membrane.

Typically, for the nanogel

decoration, two pieces of PC membranes (Diameter = 47 mm), one with pore diameter of 800 nm (PC-800) and another one with pore diameter of 220 nm (PC-220) are combined together.

As shown in Figure 6a and 6b, the PC-800 membrane



exhibits porous structures with straight pores.

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By filtrating the swollen PNB

nanogels through the PC-800 membrane at a temperature lower than VPTT, the PNB nanogels can be well remained in the pores of PC-800 membrane, due to the holding up of nanogels by the PC-220 membrane (Figure 1a,b).

After the PDA coating for

nanogel adhesion, the PC-800 membrane is decorated with PNB nanogels (Figure 1c). The SEM images in Figure 6c-f confirm the successful immobilization of PNB nanogels on the top surface (Figure 6c,d) and pore surface (Figure 6e,f) of the PC-800 membranes after PDA coating.

To construct the membrane detector for Pb2+

detection (Figure 2), the nanogel-immobilized membrane is incorporated into a filter covered with a heating jacket, and then integrated with a microfluidic control system, a flowmeter, and a buffer tank. Nanogel-Immobilized Membrane Detector for Ultrasensitive Detection of Trace Pb2+.

The ability of nanogel-immobilized membrane detector for

ultrasensitive detection is based on volume phase transitions of PNB nanogels in response to different [Pb2+] values.

Such [Pb2+]-dependent volume changes allow

manipulation of the size of membrane pores to convert [Pb2+] signals into simply-detectable trans-membrane flux changes.

When flowed with Pb2+-containing

solution, the PNB nanogels in the membrane detector swell due to the complexation between the crown ether groups and Pb2+.

This swelling of nanogels decreases the

pore size and leads to reduced trans-membrane flux.

For detection of [Pb2+], first,

the trans-membrane fluxes (J) at different temperatures and [Pb2+] are measured to determine the optimal detection temperature (Figure 7a).


With increasing

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temperature, the values of J at different [Pb2+] all increase due to the thermo-induced shrinking of PDA-coated PNB nanogels.

Meanwhile, with fixed temperature, the

value of J decreases with increasing [Pb2+] from 0 mol L-1 to 10-6 mol L-1, due to the Pb2+-induced swelling of PDA-coated PNB nanogels.

The Pb2+-induced change of J

is evaluated by using Pb2+-induced flux-changing ratio (R2), which is defined by Equation (2):

R2 = where

J Pb 2+

and

J water

J Pb2+

(2)

J water

are the trans-membrane flux of nanogel-immobilized

membrane in Pb2+-containing solution with a certain [Pb2+] and in pure water at the same temperature, respectively. The R2 first decreases and then increases with increasing temperature from 30 oC to 50 oC, showing a minimum value at 40 oC (Figure 7b).

This tendency means that, at

40 oC, the [Pb2+] change can lead to largest decrease of trans-membrane flux.

This

result is coincident with that the PDA-coated PNB nanogels can achieve largest Pb2+-induced volume swelling at 40 oC (Figure 5b).

Moreover, at 40 oC, even in

Pb2+-containing solution with [Pb2+] as low as 10-10 mol L-1, the membrane detector can still exhibit an obvious Pb2+-induced reduce of trans-membrane flux with R2 = 0.88.

Such a flux reduce can be easily detected by the flowmeter, indicating that the

[Pb2+] detection limit is as low as 10-10 mol L-1.

Based on the [Pb2+]-dependent R2

values at 40 oC (Figure 7b), the quantitative relationship between the R2 value and [Pb2+] can be obtained (Figure 7c) and described by Equation (3):  Pb 2 +  = 72.83e −4.74 R2


(3)


Page 18 of 40

Thus, the [Pb2+] value can be simply calculated by using R2 values based on the measured trans-membrane flux of the Pb2+-containing solution and pure water at 40 o

C.

Thus, the membrane detector enables highly-sensitive detection of trace Pb2+.

Based on these results, for Pb2+ detection, aqueous solution without Pb2+ can be first injected through the membrane detector via a pump at 40 oC, followed with injection of Pb2+-containing sample solution.

Then, the value of flow rate change after the

solution switch can be simply measured by a flowmeter, and then easily converted into [Pb2+] values based on the simple and quantitative relationship between flow rate changes and [Pb2+] for Pb2+ detection.

The [Pb2+] values based on flow rate changes

can be directly read by using self-made and low-cost App on smart phone, requiring no professional data analysis, as we demonstrated in our previous work.40 Moreover, the membrane detector based on flowing solutions can be directly connected into the solution-flowing pipelines such as water pipelines for real-time online detection of Pb2+ pollution.

Nanogel-Immobilized Membrane Detector for Highly-Selective Detection of Trace Pb2+.

Based on the high selectivity of PNB nanogels for recognizing Pb2+,40

the nanogel-immobilized membrane detector enables highly-selective Pb2+ detection. The highly-selective Pb2+ detection is investigated by using aqueous solutions, each containing one type of interfering ions for trans-membrane flux measurement.

To

evaluate the trans-membrane flux changes induced by different ions, the ion-induced flux change (∆J), defined by Equation (4), is used.

∆J = Jwater - Jion


(4)

Page 19 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


where Jwater and Jion are the trans-membrane flux of water (210 µL min-1) and ion-containing solution with ion concentration of 10-6 mol L-1, respectively.

The

Jwater and Jion are all measured at 40 oC under a constant pressure of 0.015 MPa.

As

shown in Figure 7d, the ∆J induced by each of the ions, including K+, Na+, Ba2+, Ca2+, Mg2+, and Sr2+, are all less than 9 µL min-1.

Meanwhile, the ∆J induced by Pb2+ is

96.5 µL min-1, which is much larger than those induced by other ions, due to the higher binding constant between 18-crown-6 groups and Pb2+ as compared with other ions.47

Generally, the formation and stability of the complexes between 18-crown-6

groups and the metal ions are dominated by the size/shape matching between the cavity of 18-crown-6 groups and metal ions.

According to the literature,48 the order

of complex constant of 18-crown-6 with metal ions is: Pb2+ > Ba2+ > other ions (such as K+, Na+, Ba2+, Ca2+, Mg2+, and Sr2+); thus, the swelling extent of PNB nanogels as well as the ∆J induced by Ba2+, are larger than other interference ions, but still negligible as compared with Pb2+.

All the results show the high selectivity of the

membrane detector for Pb2+-detection.

Moreover, for more complex samples that

contain various pollutants such as ions, microparticles, oil, proteins and microorganisms, pre-purification of the sample solutions by methods such as filtration is required to avoid blocking of the membrane pores by pollutants.

Nanogel-Immobilized Membrane Detector for Repeatable Detection of Trace Pb2+.

For Pb2+ detection, the conversion of [Pb2+] to simply-detectable flux changes

is achieved by using the responsive volume phase transitions of PNB nanogels for controlling the size of membrane pores.

To demonstrate such a control on the pore



Page 20 of 40

size for flux regulation, here the reversible swelling/shrinking volume changes of PNB nanogels in the membrane detector is simply achieved by using heating (50 oC) and cooling (10 oC) cycles (Figure 8a).

Under a constant pressure of 0.015 MPa, the

J values of pure water show repeated changes during the heating/cooling cycles. The ratio of water flux at 50 oC to that at 10 oC remains ~40 within eight heating/cooling cycles, indicating the firm immobilization of PNB nanogels in the membrane detector and their excellent performance for flux control.

Since such

reversible volume changes can also be realized when responding to different [Pb2+] changes,40 the membrane detector can show excellent performance for repeated [Pb2+] detection.

Moreover, to demonstrate the good reproducibility of our strategy for

fabricating the membrane detector, the performances of nanogel-immobilized membrane detectors fabricated from different batches for detection of trace Pb2+ are also studied.

As shown in Figure 8b, at 40 oC, the membranes fabricated from three

batches all exhibit a similar flux at [Pb2+] with a fixed value.

These results indicate

good repeatability of the membrane detector for detecting trace Pb2+, and good reproducibility of our strategy for fabricating the membrane detector.

CONCLUSIONS In summary, a novel membrane detector is created by a facile strategy combining commercialized

membranes

and

smart

nanogels,

highly-selective detection of trace threat substances.

for

ultrasensitive

and

The membrane detector is

fabricated by simply immobilizing smart nanogels onto the multiple nanochannels of


Page 21 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


pores in a commercialized membrane based on filtration and PDA adhesion.

This is

demonstrated by incorporating Pb2+-responsive nanogels in the straight pores of commercialized PC membrane for ultrasensitive and highly-selective real-time detection of trace Pb2+, with detection limit as low as 10-10 mol L-1.

Future study

needs to focus on the detection performances and anti-fouling properties of the membrane detector for more complex real samples, and on the size match between nanogels and pores for further improving the responsive flow rate change as well as the detection performances.

Based on the diverse stimuli-responsive materials for

creating smart nanogels,42,49-53 the nanogel-immobilized membrane detector, together with our simple fabrication strategy, provides a flexible platform for creating versatile new

smart-nanogel-immobilized

membrane

detectors

for

ultrasensitive

highly-selective detection of different trace threat substances.

and

The membrane

detectors show great power for myriad applications such as disease treatment and environment protection.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (L.-Y. C.)

*E-mail: [email protected] (W. W.) Author Contributions The manuscript was written through contributions of all authors. given approval to the final version of the manuscript. 21 ACS Paragon Plus Environment

All authors have


§

P.-J. Y. and F. H. contributed equally to this work.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS

The authors gratefully acknowledge support from the National Natural Science Foundation of China (21490582, 21576167), the Program for Changjiang Scholars and Innovative Research Team in University (IRT15R48) and the State Key Laboratory of Polymer Materials Engineering (sklpme2014-1-01).

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Page 32 of 40

Figures

Figure 1. Schematic illustration of fabrication of nanogel-immobilized membrane for Pb2+ detection.

(a-c) Decoration of PNB nanogels onto the PC-800 membrane via

filtration (a,b) and PDA-based immobilization (c).

(d) Isothermal swelling and

shrinking changes of PNB nanogels in response to [Pb2+] changes.

(e,f)

Pb2+-induced flux changes based on the Pb2+-responsive swelling (e) and shrinking (f) of PNB nanogels in the pores of PC-800 membrane.


Page 33 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2. Set-up of the Pb2+-detecting membrane detector based on the nanogel-immobilized membrane.



Figure 3. Compositional and morphological characterization of PNB nanogels. FT-IR spectra of PNIPAM, PNA and PNB nanogels. nanogels.

Page 34 of 40

(a)

(b) AFM image of dried PNB

The scale bar is 1 µm.


Page 35 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4. Pb2+-responsive volume phase transitions of PNB nanogels.

(a)

Temperature-dependent diameter changes of PNB nanogels in pure water and Pb2+-containing solutions with different [Pb2+].

(b) Temperature-dependent R1

changes of PNB nanogels in Pb2+-containing solutions with different [Pb2+].



Page 36 of 40

Figure 5. Pb2+-responsive volume phase transitions of PDA-coated PNB nanogels. (a) Temperature-dependent diameter changes of PDA-coated PNB nanogels in pure water

and

Pb2+-containing

solutions

with

different

[Pb2+].

(b)

Temperature-dependent R1 changes of PDA-coated PNB nanogels in Pb2+-containing solutions with different [Pb2+].


Page 37 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 6. Morphological characterization of blank and nanogel-immobilized PC-800 membrane.

(a,b) SEM images of the top surface (a) and cross-section (b) of blank

PC-800 membrane.

(c-f) SEM images (c,e) and magnified SEM images (d,f) of the

top surface (c,d) and cross-section (e,f) of nanogel-immobilized PC-800 membrane. Scale bars are 5 µm in (a-c, e) and 2 µm in (d, f).



Page 38 of 40

Figure 7. Nanogel-immobilized membrane detector for ultrasensitive and highly-selective detection of trace Pb2+.

(a) Effects of temperature and [Pb2+] on the

trans-membrane flux changes of the membrane detector.

(b) Temperature-dependent

R2 changes of the membrane detector at different [Pb2+]. relationship between [Pb2+] and R2.

(c) Quantitative

(d) Flux changes of the membrane detector

induced by different ions at 40 oC.


Page 39 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 8. Nanogel-immobilized membrane detector for repeated Pb2+ detection.

(a)

Flux changes of the membrane detector upon repeated heating/cooling.

(b)

[Pb2+]-dependent trans-membrane fluxes of membrane detectors constructed from different batches.



Graphic for TOC


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A Novel Membrane Detector Based on Smart Nanogels for

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