High-Sensitivity Detection of Iron(III) by Dopamine-Modified Funnel

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Functional Nanostructured Materials (including low-D carbon)

High-Sensitive Detection of Iron (III) by Dopamine-Modified Funnel-Shaped Nanochannel Bo Niu, Kai Xiao, Xiaodong Huang, Zhen Zhang, Xiang-Yu Kong, Ziqi Wang, Liping Wen, and Lei Jiang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b05686 • Publication Date (Web): 11 Jun 2018 Downloaded from http://pubs.acs.org on June 11, 2018

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ACS Applied Materials & Interfaces

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High-Sensitive Detection of Iron (III) by

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Dopamine-Modified Funnel-Shaped Nanochannel

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Bo Niu1,2,†, Kai Xiao1,†, Xiaodong Huang1, Zhen Zhang1, Xiang-Yu Kong1,*, Ziqi Wang1,

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Liping Wen1,2,* and Lei Jiang1,2

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1

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and Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China.

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2

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R. China

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*

Key Laboratory of Bio-inspired Materials and Interfacial Science, Technical Institute of Physics

School of Future Technology, University of Chinese Academy of Sciences, Beijing 100049, P.

Corresponding Author

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E-mail: [email protected]; [email protected]

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†

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Keywords: funnel-shaped nanochannels, dopamine, FeIII sensor, EDTA-2Na, switchability

These authors contributed equally to this work.

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ACS Paragon Plus Environment

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Abstract

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Iron as an essential trace element in human body participates in various biological processes.

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The demand for efficient and sensitive detection of FeIII has drawn wide attentions. Inspired by

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biological nanochannels, a high-sensitive, economic and recyclable FeIII detection method is

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proposed by using dopamine-modified funnel-shaped nanochannels. Along with the FeIII

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concentration changing, different FeIII-dopamine (FeIII-DOPA) chelates are generated in the

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channel, which affect the wettability and charge distribution of the pore surface, resulting in the

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changing of ionic current through the nanochannels. Meanwhile, the funnel-shaped nanochannel

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applied in this work with a narrow cylindrical segment (diameter close to 10 nm) as the critical

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section can enhance the sensing ability to ultra-trance level (down to 10-12 M). We expound the

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mechanism and feasibility of this method, and anticipate the system can be a good example to

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design highly sensitive and stable ion detection devices.

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Introduction

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FeIII is a biological important metal ion and plays essential roles in synthesizing a series of

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enzymes involved in oxygen uptake, oxygen metabolism, and electron transfer.1-2 Imbalanced

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FeIII levels affect the normal activities in living organisms severely.3-5 For example, excessive

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FeIII in human body is poisonous, and only when the high valence FeIII uptake from the drinking

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water and food is converted to low valence states FeII, can it be absorbed by intestinal tract. Once

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the FeIII is over intaken, the insolubility of FeIII and the formation of toxic radicals, especially the

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hydroxyl radical, will cause severe disease. Therefore, regular circulation of FeIII has become an

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important indicator of human health status, which makes it particularly necessary to demonstrate

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the further research of FeIII detection technology.6 Currently, some methods involved in the

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detection of FeIII have been implemented by designing fluorescent probes7-9, functionalizing

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doped carbon nanomaterials10-12 and graphene quantum dots13-15 and synthesising of metal-

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organic-frameworks16-18, etc. Those methods have realized the sensitive detection of FeIII, and

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some of them exhibited specific recognition. However, these technologies are always costly or

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have limited detection limits, which restrict their further applications. Recently, we and other

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groups reported that functionalized nanochannels could be used as a new platform to realize

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various ions detection.19-22 Despite the simply structure, the sensing devices based on the

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biomimetic nanochannels have achieved the highly specific recognition and ultrasensitive

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detection.23-25 The fabricated nanochannels exhibit excellent characteristics such as controlled

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shape and pore size, good stability, and ease of incorporation with other devices.26-27 These

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excellent properties make bionic nanochannels be a potential substitute for biological channels.

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To date, the main research focused on the design of the different shapes and the modification of

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functional molecules in the nanochannels. In detail, various solid-state nanochannels with


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different morphology including symmetrical shapes, such as cylindrical,28 hourglass-shaped,29

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cigar-shaped30 nanochannels and asymmetric shapes, such as conical,31 bullet-shaped32 and

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funnel-shaped33 nanochannels have received extensive attention. Among all the different shapes

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of nanochannels, the funnel-shaped nanochannel which possess a straight cylindrical segment as

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the critical section, has dramatically improved the controllability and the stability in ion

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transport.33 Based on this, a series of experiments to verify the special characteristics of funnel-

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shaped nanochannel has been carried out recently.34-35

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In this work, we report the dopamine-modified funnel-shaped nanochannels that can realize

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recyclable ultra-trace FeIII ions detection. As shown in Figure 1a, the DOPA modification can be

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accomplished by one step coupling reaction between the amino in DOPA and the carboxyl

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groups in the inner surface of the channel, which could be confirmed by X-ray photoelectron

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spectrum and contact angle measurement (Figure S1&S2, supporting information). Then,

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different FeIII-DOPA chelates were generated along with the ratio of DOPA and FeIII.36 At last,

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by adding disodium ethylenediaminetetra-acetate (EDTA-2Na), the FeIII-DOPA chelates were

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destroyed and FeIII was removed from the channel because the binding capacity of FeIII to

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EDTA-2Na was stronger than that of FeIII with DOPA (Figure 1 iv, v).37 Based on this, such a

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system can clearly realize the recyclable FeIII ions detection. This particular funnel-shaped

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nanochannel used in this work was shown in Figure 1b, which had two different segments: the

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conical section with ~7.5 µm length and the cylindrical section with ~4.5 µm length. The

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lengthened straight critical section with diameter of ~10 nm was benefit to improve the

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sensitivity of ion identification.


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Figure 1. (a) The schematic description of the FeIII activated ionic nanochannel and the inner

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surface of the funnel-shaped nanochannels at different states: the unmodified state after etching

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(i); DOPA modified (ii); cross-linked chelate after FeIII response with excess DOPA (iii); cross-

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linked chelate after equal FeIII response with DOPA (iv); and the EDTA-2Na treatment (v). (b)

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The cross-section SEM images of a single funnel-shaped PET nanochannel with conical segment

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and cylindrical section.

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Results and Discussion

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The current-voltage (I-V) as a current value measurement is extensively applied in verifying

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the properties of the fabricated nanochannel including the charge distribution and wettability of

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the inner surface, even calculating the channel (pore) radius.38-39 As shown in Figure 2, the I-V

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curves provide direct signals to demonstrate the successful modification of DOPA. In particular,

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each step of reaction was monitored by measuring I-V in 0.1 M KCl. The DOPA modified


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funnel-shaped nanochannel showed different behaviours under different pH. As shown in Figure

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2a, when the pH value was 3, the unmodified nanochannel showed symmetric ion transportation

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properties. The ion current value (absolute value) at ± 2V was almost the same (2.4 nA at 2 V; -

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2.5 nA at -2 V), which indicated there was no rectification (the rectification was defined as the

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ratio of absolute current at given negative voltage and the current value at corresponding positive

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voltage). After the nanochannels were modified with DOPA, the current at positive voltage

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increased intensively, and the rectification occurred, which were attributed to the protonation of

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the hydroxyl groups. The I-V curves decreased to the unmodified level after immersing the

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DOPA-modified nanochannels in 20 mM FeCl3 solution for 2 h. This phenomenon was

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contributed to the less surface charge, which was caused by the FeIII locating in the centre of the

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chelates and the hydrophilic hydroxyl group decreased remarkably during the reaction. In

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addition, the contact angle measurement showed a more hydrophobic surface after adding FeIII,

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which confirmed the chelating process. (more information see Figure S2). As shown in Figure

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2b, under pH 7, the DOPA modified nanochannel showed negative surface charges due to the

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carboxyl formed in the etching process. After the responded to FeIII, the ion currents decreased

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further, to about -8.2 nA under -2 V and about 3.4 nA under +2 V. For the different states:

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unmodified, DOPA modified and DOPA-FeIII modified, the corresponding ion current values at -

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2 V were -45.2 nA, -22.3 nA and -8.4 nA, respectively. The values showed a decreasing trend,

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which was mainly due to the reduction of the surface negative charge and the decrease of

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wettability.


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Figure 2. (a) The I-V curves of the nanochannels before (black), after (navy) DOPA

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modification and FeIII response (dark cyan) in pH 3, 0.1 mol L-1 KCl solution and the scheme of

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DOPA modified funnel-shaped single nanochannel under pH 3. (b) The I-V curves of the

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nanochannels before (black), after (navy) DOPA modification and FeIII response (dark cyan) in

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0.1 mol L-1 of KCl solution pH 7 and the scheme of DOPA modified funnel-shaped single

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nanochannel under pH 7.

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As shown in Figure 2, the I-V curves of dopamine modified nanochannel had a response of pH.

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At pH 3 and 7, the nanochannel showed opposite rectification, which indicated the charge in the

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nanochannel realized a reversion from positive to negative. Based on the reversion of the charge,

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there would be an “isoelectric” point between pH 3&7. Therefore, we carried out the experiment

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of different pH to find out the “isoelectric” point. As shown in Table 1, we chose 7 points of


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different pH values from 2.60 to 5.63. The rectification ratios calculated by I+2V/I-2V showed

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remarkable decrease as the pH increased from 2.60 to 4.99. It was worth mentioning that the

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rectification ratio decreased to 1.00 when the pH values were close to 4.99. As the pH continued

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to increase to 5.63, the rectification ratio (calculated by |I+2V/I-2V|) gradually increased from 1.05

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to 3.56. In order to clearly describe the “isoelectric” point, the I-V curves and the distribution of

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rectification ratios along with the pH were presented in Figure 3. As shown in Figure 3a, the I-V

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curves of the DOPA-modified nanochannel at pH 2.60, 4.99 and 5.63 exhibited three different

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curves. When the pH was 2.60, the nanochannel was positively charged, thus, the I-V curve

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showed promoted ion current at positive voltage. While the pH was increased to 5.63, the

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nanochannel was negatively charged and the I-V curve showed restricted ion current at positive

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voltage. More noteworthy, the I-V curve in Figure 3a was a “symmetrical line” at positive and

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negative voltage when the pH was 4.99 (dark cyan). As described above, the “isoelectric” point

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meant the charge in the nanochannel was balanced, thus, the rectification ratio was closed to 1.

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As shown in Figure 3b, When the pH was at 4.85 and 4.99, the rectification ratio fell on the line

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of x = ± 1. (The algorithm |I-2V/I+2V| and I+2V/I-2V made the points fall around the edge of the

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light blue area of Rec = ± 1) The zwitterionic nature of dopamine led to the rectification reversal

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below and above the isoelectric point (pH=4.9).

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Table 1. The ion current values at ± 2V under different pH conditions and the rectification ratios

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calculated by| |I-2V/I+2V| (I+2V/I-2V).

pH I-2V /A× ×10-8 I+2V/A× ×10-8 R(I+2V/I-2V) R(| |I-2V/I+2V|)

2.60 -3.95 57.86 -14.66 /

3.62 -3.02 29.54 -9.78 /

4.63 -6.10 12.00 -1.97 /

4.85 -10.54 10.96 -0.96 1.04

4.99 -4.15 4.38 -0.95 1.05

5.23 -3.94 2.49 / 1.58

5.63 -14.65 4.12 / 3.56

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Figure 3. (a) The I-V curves of the DOPA-modified nanochannel under different pH values (3.62,

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4.99 and 5.63). (b) Rectification ratio (Rec) as a function of pH.

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To clearly figure out the detailed mechanism of the reaction between the DOPA and FeIII, the

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experiment between different concentration FeCl3 solutions and quantitative DOPA was carried

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out. In general, metal complexes, especially the chelates, show characteristic intense colours and

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the colour changes with different metal concentrations.40-41As shown in Figure 4, the specific

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FeIII responding process was characterized through the colour change in solution. At first, the

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DOPA dissolved in water was colourless. With the continuous addition of FeCl3, the colour of

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the solution gradually deepened to dark green. After adding EDTA-2Na, the dark green solution

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turned back to colourless again. The colour changes were caused by the complicated reaction

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between DOPA and FeIII. In detail, one FeIII may bind more than one DOPA molecule. The

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bonding reaction happens between the two functional groups (phenolic hydroxyl) and FeIII

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(Figure S3). When the FeIII was added into the DOPA overdosed solution, a cross-linked

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complex [Fe(DOPA)3] with FeIII : DOPA= 3 : 1 was formed, along with the continuously adding


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FeCl3 ,the complex Fe(DOPA) with FeIII : DOPA= 1 : 1 was formed.36,42 The different structures

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of the chelates exhibited different colours in the solution.43

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Figure 4. Schematic illustration of colour change of the DOPA solution mixed in different

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concentration of FeIII, from which we can obtain the probably products of the different ratio of

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DOPA and FeIII. Moreover, the addition of EDTA-2Na removing the color in the vials directly

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demonstrates that the bond force of EDTA-2Na with FeIII is stronger than that of DOPA with

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FeIII.

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Meanwhile, UV-vis absorption spectra of different ratio of FeIII and DOPA in solvent were

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collected. As expected, the UV-vis absorption spectra of DOPA and FeIII at different ratios

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provided detailed mechanism of FeIII reacting with DOPA (Figure 5). DOPA solution (20 mM)

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exhibited one broad absorption band ranging from 250 to 320 nm with a maximum located at

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λmax=280 nm, which resulted from the two phenolic hydroxyl.44 Then, the UV-vis absorption


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intensity of DOPA was enhanced gradually with increasing FeIII concentrations. In particular, the

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absorption peak of the hydroxyl group could be measured when no FeIII was added or the ratio of

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DOPA to FeIII was less than 1:2. With the increasing of FeIII ions, the amount of functional group

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(phenolic hydroxyl) decreased significantly, thus, the characteristic absorption peak gradually

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disappeared. (the light blue section overlaid between the λ=260 & 305 nm clearly showed the

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change of the absorption peak)

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Figure 5. The UV-vis absorption spectrum (λ=200 nm~500 nm) by fresh 20 mM DOPA (keep

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away from light, 0.1 M Tris-HCl buffer) and FeCl3 solution at different ratio.

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As mentioned above, different chelates can be formed with altering ratio between DOPA and

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FeIII. As shown in Figure 6a, the DOPA-modified nanochannels can even response to FeIII ions at

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the ultralow concentration (0.01 nM), which proved that the DOPA-modified nanochannel was

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extremely sensitive to FeIII ions. The I-V curves of full FeIII concentration from 0.001 nM to 100

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nM can be seen in the Figure S4. The ion current increased from -163.3 nA to -46.4 nA gradually

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along with the increase of FeIII concentration. When the concentration of FeIII reached to 1 nM,


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the ionic current possessed a maximum value of -46.4 nA at -2 V. The trend of the ion current

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was indicated on the side of the I-V curves in Figure 6a. With the further increase of FeIII

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concentration, the ionic current had a tendency to decrease from -46.4 nA to -143.2 nA at -2 V

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and increase from 11.4 nA to 23.7 nA at +2 V (Figure 6a, b). A schematic presentation for this

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phenomenon was shown in Figure 6c (i, ii, iii, iv). The three schemes in Figure 6c (ii, iii and iv)

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correspond to the three vertices in Figure 6b, respectively. As shown on the simulated nanopore

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surface, the charge distribution and the wettability of the nanochannels have a significate impact

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on the ion transport. Figure 6c (i) showed the state of the DOPA modified nanochannels. When

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the FeIII concentrations was low (10-3 nM), the nanochannel was partially negative charged

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because not all hydroxyl can combine with FeIII ions (Figure 6c ii). Along with the increasing of

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FeIII concentration to 1 nM, the chelates can be formed between three DOPA molecules and one

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FeIII ion. Under this condition, the nanochannel was electronic neutral and low hydrophilicity

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(Figure 6c iii). With the further increase of FeIII to 100 nM, the complex DOPA-FeIII chelates

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can be formed between one DOPA molecular and one FeIII ion. Under this condition, the pore

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surface was negatively charged due to the free negative hydroxyl bonding to FeIII.45 The change

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of the surface charge density and the wettability were the essential reason for the changing of

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current values.


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Figure 6. FeIII response to DOPA-modified funnel-shaped nanochannels. (a)The I-V curves of

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the DOPA-modified nanochannel with increasing FeIII concentration. (b) With the increasing

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FeIII concentration, the change trend of current was observed at -2 V and +2 V. (c) Schematic

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illustrations of the wettability and charge distribution changes in the nanochannel.

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To clearly figure out whether the sensitivity of the FeIII detection by funnel-shaped bioinspired

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nanochannel is based on the enhanced critical section, we fabricated the widely studied conical-

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shaped nanochannels, which possess a tip point not a cylinder as the critical section. The DOPA-

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modified conical-shape nanochannel was used as a control experiment. As shown in Figure 7a,

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when the concentration of FeIII was 0.01 nM, the I-V curve coincided with the DOPA-modified I-


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V curve. That’s to say, the conical-shaped nanochannel cannot recognize the ultralow level FeIII

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(Table.S1). Hereafter, we increased the concentration of FeIII to 10 nM, the ions current value

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experienced a slight fluctuation: from -22.7 nA to 25.4 nA. With the further increasing of the

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FeIII concentration, the ions current value at ± 2 V tended to be stable (Figure 7b). Normally, the

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nanochannels with small ion current values were more sensitive. The conical nanochannel was

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etched with smaller pore size in this experiment (Figure S5). However, the response to FeIII had

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obvious weakness compared with the funnel-shaped nanochannel, which provided strong

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evidence that the ultra-sensitive detection ability did come from the function of the extended

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critical section.

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Figure 7. FeIII response in DOPA-modified conical-shaped nanochannels. (a)The I-V curves of

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the DOPA-modified conical-shaped nanochannels with increasing FeIII concentration. (b) With

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the increasing FeIII concentration, the change trend of ion current was observed at ± 2 V.

231


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232

Table 2. Methods involved in the sensitive detection of FeIII by different technology in recently

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years. Materials (Methods)

LOD (M)

Reference

Luminescent Cd(II)–Organic Frameworks with Chelating NH2 Sites

2.7×10-6

[18]

Fluorescent Boron Nitride Quantum Dots

3.0×10-7

[8]

Zr(IV)-Based Metal−Organic Frameworks

2.8×10-7

[17]

Dendrimer-Based Electropolymerized Microporous Film

8.5×10-8

[47]

Rhodamine-Functionalized Graphene Quantum Dots

2.0×10-8

[13]

Luminescent N,S Co-doped Graphene Quantum dots

8.0×10-9

[15]

Nitrogen and Phosphorus Co-Doped Carbon Nanodots

1.8×10-9

[12]

Ligninsulfonate/graphene Quantum Dots Composites

5.0×10-10

[14]

Fluorescent Organic Nanoparticles

1.0×10-10

[7]

π-Plasmon Absorption of Carbon Nanotubes

1.0×10-11

[11]

234 235

With an extended cylindrical segment, the funnel-shaped nanochannel achieved ultra-sensitive

236

detection of FeIII. Previously, many works involved in the sensitive detection of FeIII were

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reported. Among them, the synthesis fluorescent probes are thoroughly discussed and

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comprehensive applied in FeIII detection. Besides the fluorescent probes, the new methods such

239

as MOFs, Films46, carbon nanotubes, graphene quantum dots and so on, have gradually

240

developed. As shown in Table 2, we summarized the articles published in the past two years on

241

the detection of iron ions. The different methods, materials and limits of detection are listed in

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the table. Compare to the system in this article, the simple structure is one of its major

243

advantages. Unlike the artificial bioinspired nanochannels, the complex process and expensive

244

cost of sensitive probes or carbon materials make the FeIII detection technology cannot be


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promoted. Thus, the funnel-shaped nanochannels modified with functionalized molecule will

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find application in the ultra-sensitive ion detection.

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Figure 8. Stability and repeatability of the FeIII activated ionic nanochannel. (a) The reversible

248

ionic currents before and after removing of FeIII by adding EDTA-2Na. (b) Stability and

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responsive switchability of the DOPA-modified nanochannels at -2 V.

250

FeIII activated ionic nanochannel also exhibited good stability and responsive switchability,

251

which could be demonstrated by the addition and removal of FeIII. Figure 8a showed the I-V

252

curves of the DOPA-modified nanochannels before and after immersing in solution and then

253

treated with EDTA-2Na respectively. By immersing in FeCl3, the nanochannel changed from “on”

254

(relatively high ion current value: -38.1 nA at -2 V) state to “off” (ion transport was restricted

255

and showed low ion current value: -6.2 nA at -2 V) state. When EDTA-2Na was added into the

256

system, the I-V curve returned to the “on” state. As the binding capacity of EDTA-2Na to FeIII

257

was stronger than that of DOPA with FeIII, EDTA-2Na could take FeIII from DOPA-FeIII cross-

258

linked chelates47 and the ionic current recovered to the initial DOPA-modified state. As shown in

259

the Figure 8b, ON/OFF states switched upon alternating introduction of DOPA and EDTA-2Na,


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and no damping of the ionic current was observed after several cycles, which reflected the high

261

reversibility and repeatability of the system.

262

Conclusion

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In summary, an ultra-low concentration of FeIII detection method was successfully realized by

264

immobilizing DOPA in funnel-shaped nanochannels. Owing to the special structure, the funnel-

265

shaped nanochannel combining with functional molecule provided a new platform to detect trace

266

amounts of FeIII ions. Specifically, this system can directly realize the ultra-trance FeIII detection

267

with concentration of 10-3 nM. Furthermore, with the excellent stability and switchability of the

268

funnel-shaped nanochannels, our proposed platform may have potential applications in high

269

sensitively ion detection fields, such as biological toxicity examination and water quality

270

monitoring.

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Experimental Section

272

Materials: The reagents used in this experiment were all analytically grade. NaOH, KCl,

273

HCOOH,

N-(3-Dimethylaminopropyl)-N-ethylcarbodiimide

hydrochloride

(EDC),

N-

274

Hydroxysul fosuccinimide sodium salt (NHS), DOPA, Tris-HCl, FeCl3, EDTA-2Na were

275

purchased from Sinopharm Chemical Reagent Beijing Co., Ltd. (SCRC, China) and J&K Beijing

276

Co., Ltd. Poly(ethylene terephthalate) (PET) membranes (thickness of 12 um) (HostaphanRN12,

277

Hoechst, density of 106 cm-2) were irradiated at the linear accelerator UNILAC (GSI, Darmstadt)

278

with swift heavy ions (Au) which have an energy of 11.4 MeV per nucleon. The fabrication of

279

nanochannels in the PET membrane was accomplished by asymmetric etching of the damage

280

trails which were coursed by the heavy ions (Au, Bi or U) passing through the membrane. Before


17


Page 18 of 27

281

the chemical etching process, the PET films were exposed to UV light (365 nm, 20 W) for 1 h

282

from each side to improve the etching properties.

283

Fabrication of funnel-shaped nanochannel: The funnel-shaped nanochannels were prepared

284

by an asymmetric track etching technology on a PET membrane containing 106 nanopores/cm2,

285

approximately. Concentrated NaOH has been selected as the etchant reagent for preparing bio-

286

inspired nanochannels on PET membrane. At first, the PET membrane was irradiated with single

287

swift heavy ions (Au) to obtain damage trail; then, the membranes were irradiated under UV

288

light (365 nm) for 2 h from both sides in order to make the film more conducive to etching; last,

289

start etching. The specific process were as follows: the etching (NaOH, 9 M) and the stopping

290

solution (1 M KCl and 1 M HCOOH) are added in two different sides of the reaction cell (Figure

291

S6).

292

Owing to its special construction, this process was carried out by two steps: one for conical

293

section and the other for cylinder section. The Pt electrode was used to detect the current change

294

under the voltage 1 V during the etching process. When the transmembrane ionic current

295

achieved an increasing from 10-12 A to 10-9 A, which meant several of the nanochannels opened

296

and the conical part had been presented. Remove the membrane quickly and switch to a 60 °C

297

reaction pool to continue etching. The etching (2 M NaOH) and stop solution (1 M KCl and 1 M

298

HCOOH) were both heated to 60 °C. In particular, the cylindrical portion was obtained by

299

etching on the same side. Owing to the temperature have more profound impact than the

300

concentration of the etching solution, generally, the higher temperature process lasted for 4~5

301

minutes. Wait until the ion currents reach to a certain number, then stop etching. Remove the

302

etching solution and add the stop solution to the pool quickly. It was worth noting that the NaOH

303

could not be cleaned completely, thus, washing the pool more than three times was very


18

Page 19 of 27 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


304

necessary. The fabrication of the funnel-shaped nanochannel would finish until the current was

305

stable. Finally, the prepared membrane was immersed in Milli-Q water (18.2 MΩ) for next step.

306

The big size pore (the radius of base side approximate to 400 nm) of the nanochannels can be

307

observed under scanning electron microscopy. The small size pore (the radius of tip side

308

approximate to 10 nm) can be observed in the profile.

309

Functionalization with DOPA: The functional molecular DOPA was immobilized on the inner

310

surface of the funnel-shaped nanochannel by chemical modification. After the ionic trail-etching,

311

a large number of carboxyl (–COOH) functional groups were produced in the inner face of the

312

funnel-shaped nanochannel. Above all, the PET membranes were activated in mixture solution (2

313

mL) consist of EDC (30 mg) and NHS (6 mg) for 1 hour. In this process, the –COOH were

314

activated into a more easily reactive group. Single DOPA molecular was modified in the

315

multiple nanochannels at acidic solution (dopamine 20 mM, 0.1 M Tris-HCl buffer, pH 5.7)

316

mainly because acidic conditions can effectively inhibit the self-polymerization of DOPA

317

molecules, so as to ensure that the modification in the pore is monomer DOPA. The modification

318

was accomplished by one step coupling reaction of the amino in DOPA and the carboxyl acid in

319

surface. Chemical modification cannot modify all of the activated carboxyl groups with DOPA

320

due to the inevitable byproduct. After modification, the membrane was washed with Milli-Q

321

water (18.2 MΩ) for more than three times.

322

Functionalized nanochannel chelating with FeIII ions: After functionalization with DOPA,

323

FeCl3 solution prepared in Tris-HCl was introduced on the both side of the funnel-shaped

324

nanochannels. The immobilization of FeIII lasted for 2h at room temperature. After that, the

325

membrane was washed three times and reserved for backup.


19


Page 20 of 27

326

Supporting Information

327

Experimental details about the fabrication process of funnel-shaped nanochannels and the post

328

modification of DOPA. Figures about X-ray photoelectron spectroscopy of a planar PET

329

membrane, contact angle measurement, diagram of the experimental facility, I-V curves of the

330

concentration measurement, and the reaction equation between DOPA and FeIII. Table about the

331

different methods involve in the detection of Iron ions.

332

Author Contributions

333

†

334

Notes

335

The authors declare no competing financial interests.

336

Acknowledgements

337

The authors thank the Material Science Group of GSI (Darmstadt, Germany) for providing the

338

ion-irradiated samples. This work was supported by the National Key R&D Program of China

339

(2017YFA0206904, 2017YFA0206900), the National Natural Science Foundation (21625303,

340

51673206, 21434003), and the Key Research Program of the Chinese Academy of Sciences

341

(QYZDY-SSW-SLH014).

These authors contributed equally to this work.

342 343 344


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High-Sensitivity Detection of Iron(III) by Dopamine-Modified Funnel

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