Photoelectrochemical Lab-on-Paper Device Based on an Integrated

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Photoelectrochemical Lab-on-Paper Device Based on Integrated Paper Supercapacitor and Internal Light Source Lei Ge, Panpan Wang, Shenguang Ge, Nianqiang Li, Jinghua Yu, Mei Yan, and Jiadong Huang Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/ac4001496 • Publication Date (Web): 08 Mar 2013 Downloaded from http://pubs.acs.org on March 26, 2013

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Analytical Chemistry

Photoelectrochemical Lab-on-Paper Device Based on Integrated Paper Supercapacitor and Internal Light Source

Lei Ge,a Panpan Wang,a Shenguang Ge,a Nianqiang Li,b Jinghua Yu,*a Mei Yan,a Jiadong Huanga

a

Key Laboratory of Chemical Sensing & Analysis in Universities of Shandong, School of Chemistry and Chemical Engineering, University of Jinan, Jinan 250022, P.R. China

b

School of Information Science and Engineering, University of Jinan, Jinan 250022, China

*Corresponding author: Jinghua Yu E-mail: [email protected] Telephone: +86-531-82767161

ACS Paragon Plus Environment


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Abstract In this work, photoelectrochemical (PEC) method was introduced into microfluidic paper-based analytical device (µ-PAD) and thus a truly low-cost, simple, portable, and disposable microfluidic PEC origami device (µ-PECOD) with internal chemiluminescence light source and external digital multi-meter (DMM) was demonstrated. The PEC responses of this µ-PECOD were investigated and the enhancements of photocurrents in µ-PECOD were observed under both external and internal light source compared with that on traditional flat electrode counterpart. As a further amplification of the generated photocurrents, an all-solid-state paper supercapacitor was constructed and integrated into the µ-PECOD to collect and store the generated photocurrents. The stored electrical energy could be released instantaneously through the DMM to obtain an amplified (~13 fold) and DMM-detectable current as well as a higher sensitivity than the direct photocurrent measurement, allowing the expensive and sophisticated electrochemical workstation or lock-in amplifier to be abandoned. As a model, sandwich adenosine triphosphate (ATP)-binding aptamers were taken as molecular reorganization element on this µ-PECOD for the sensitive determination of ATP in human serum sample in the linear range from 1.0 pM to 1.0 nM with a detection limit of 0.2 pM. The specificity, reproducibility, and stability of this µ-PECOD were also investigated.


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Introduction Although, as one of the most important inventions, paper has been widely used in daily life as an important medium for packaging, expression, documentation, displaying, propagation of knowledge and information, paper is already utilized extensively as a platform in analytical and clinical chemistry 1. As an analytical support, paper, being low cost (≈ 0.1 cent dm−2), high abundant, elevated porosity, disposable or biodegradable as well as being easy to use, store, transport, and printing, has excellent chemical compatibility with many applications 2. Since the first patterned paper was proposed by Whitesides and co-workers 3，paper has never attracted as much attention as it does now. The last five years have witnessed a fast progress in the field of microfluidic paper-based analytical devices (µ-PADs)

4-10

,

which represent new and outstanding approaches to truly simple, portable, disposable, and low-cost devices for molecular analysis, environmental detection, and health monitoring in developing countries, resource-limited, and remote regions. Inspired by this simple technique, many groups have paid great efforts to the development of µ-PADs, including fabrication methods for µ-PADs modifications on µ-PADs

11-23

, functionalizations or

7-10, 24-36

, and colorimetric analytical methods on µ-PADs

3,

37-54

. Recently, electrochemical and luminescent methods alone as well as their

combination have been widely employed as analytical methods on µ-PADs, such as absorbance

and

electrochemistry

fluorescence

25,

39,

55,

56

,

chemiluminescence

61-63

, electrogenerated chemiluminescence (ECL)


(CL) 64-68

57-60

,

. As a


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photo-voltaic conversion method, the principle of photoelectrochemistry has been studied extensively since the 1960s

69

construction of photocatalytic system

70

and has been utilized extensively in the and solar cells

71

. However, as a detection

method, photoelectrochemical (PEC) method is a newly developed and promising analytical method

72-75

. In the PEC detection process, light is used to excite PEC

active species on the electrode and current is used as the detection signal, which is just the reverse process of ECL. Coupling photo-irradiation with electrochemical detection, PEC sensors have the advantages of both optical methods and electrochemical methods

76

. Benefiting from the total separation of excitation source and detection

signal, its sensitivity could potentially match that of the ECL due to the greatly reduced background signal. In addition, the instrument should be simpler and lower cost than all the optical detection methods due to the use of electronic detection, particularly in an array format. Thus, this technique shows promising analytical potentials for low-cost, simple, portable, rapid and high-throughput biological assay on µ-PADs. However, to the best of our knowledge, no report about establishing PEC method on µ-PADs has been published. In all the conventional PEC methods, to measure the weak photocurrents sensitively, an electrochemical workstation

73

or lock-in amplifier

77

is required.

However, the expensive and sophisticated workstation or lock-in amplifier makes the instrument complicated and departures from the portable and low-cost trend for µ-PADs. Hence, a strategy for substitution of electrochemical workstation or lock-in amplifier is highly deserved. Recently, paper has continued to expand its applications


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beyond microfluidic devices and novel applications such as paper electronic devices have been demonstrated 78. Many efforts in the flexible electronics community have been done to employ paper as substrates for organic photodiodes 79, batteries capacitive touch pads Supercapacitor

83

, active matrix displays

84

, supercapacitor

85

51, 80-82

,

and so on.

86, 87

, a state-of-the-art circuit component that can temporarily store a

large amount of electrical energy and release it when needed have attracted much attention due to their high electrical energy storage, long life cycles, and fast charging-discharging rate. Thus, in this work, an all-solid-state paper supercapacitor was constructed and integrated into the PEC µ-PAD as effective electrical energy storage unit to collect and store the photocurrents produced by the PEC µ-PAD. The stored electrical energy could be released instantaneously through a low-cost, portable and simple digital multi-meter (DMM) to obtain an amplified current and thus a higher sensitivity than the direct photocurrent measurement, allowing the expensive and sophisticated electrochemical workstation or lock-in amplifier to be abandoned in PEC assay for the first time. In this work, a novel microfluidic PEC origami device (µ-PECOD) was designed and fabricated on a single sheet of paper based on the principles of origami and kirigami (Scheme 1) to demonstrate the integration of PEC assay and paper supercapacitor on µ-PADs. In this work, to demonstrate the feasibility of this µ-PECOD, adenosine triphosphate (ATP) was employed as a proof-of-concept analyte because the sandwich ATP-binding aptamers

88, 89

were readily available for it. To

further develop simple, low-cost, and portable PEC assay on µ-PAD, CL excited PEC



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assay

90, 91

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was adopted in this work based on N-(aminobutyl)-N-(ethylisoluminol)

functionalized gold nanoparticles

92

(ABEI-AuNPs)-H2O2 CL system using

p-iodophenol (PIP) as CL enhancer. Furthermore, it was found that emission spectrum of CL generated by the ABEI-AuNPs-H2O2 CL system overlapped the absorption spectrum of the thioglycolic acid (TGA)-capped water-soluble cadmium sulfide nanoparticles (CdS NPs), thus, CdS NPs was employed as the PEC active species to generate photocurrents. This µ-PECOD could be easily translated to detect a variety of molecules through the substitution of this model aptamer by other different recognition elements, such as aptamers antibodies

90, 93

, single-stranded DNA probes

91, 96

, and molecularly imprinted polymers

94, 95

,

97, 98

. This work contributes to

the production of low-cost, simple, and portable µ-PADs.

Experimental Section Reagents All reagents were of analytical grade and directly used for the following experiments as supplied. Ultrapure water obtained from a Millipore water purification system (resistivity ≥ 18.2 MΩcm) was used in all assays and solutions. The aptamer, split into two oligonucleotides, were purchased from Sangon Biotech Co.,Ltd. (Shanghai, China), and the sequences of the two oligonucleotides were as follows: ssDNA1, 5’- NH2-TTTTTTTTTTACCTGGGGGAGTAT-3’ ssDNA2, 5’-TGCGGAGGAAGGTTTTTT-SH-3’ ATP, cytidine triphosphate (CTP), guanosine triphosphate (GTP), and uridine triphosphate (UTP) were purchased from Aladdin Chemistry Co. Ltd.. Whatman


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chromatography paper #1 was purchased from GE Healthcare Worldwide and used with further adjustment of size (A4 size). Polyvinyl alcohol (PVA) (98-99% hydrolyzed, medium molecular weight), poly(dimethyldiallylammonium chloride) (PDDA) (20%, w/w in water, molecular weight = 200 000-350 000), and PIP were purchased from Alfa Aesar. Multi-walled carbon nanotubes (CNTs, diameter, 30-50 nm) were purchased from Nanoport. Co. Ltd. (Shenzhen, China). ABEI, 3-mercapto-1-hexanol (MCH), and N-Hydroxysuccinimide (NHS) were obtained from Sigma (St. Louis, MO). 1-Ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride (EDC) was purchased from Pierce (Rockford, IL). ABEI-AuNPs and the labeled ssDNA2 (ssDNA2-AuNPs-ABEI) were synthesized according to a previous work with modification 92. CdS NPs (5.3 nm) were prepared in water 73. The H2SO4-PVA gel electrolyte was prepared by mixing 6.0 g PVA powder with 6.0 g H2SO4 in 60 ml water and subsequent heating under stirring to ~85°C until the solution becomes clear 85, 87. Design and fabrication of this µ-PECOD In this work, wax was used as the paper hydrophobization and insulation agent to construct hydrophobic barrier on paper. The shape for wax-printing of this µ-PECOD was designed using Adobe illustrator CS4. As shown in Scheme 1A, the wax patterns of this µ-PECOD was comprised of a PEC reaction tab (red-rectangle: 20.0 mm × 15.0 mm) and a PEC collection tab (blue-rectangle: 40.0 mm×15.0 mm) with two rectangular paper legs (10.0 mm×30.0 mm). On the PEC reaction tab, there was a paper sample zone with a diameter of 6.0 mm (Scheme 1B). Between the PEC



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reaction tab and PEC collection tab, the unprinted line (1 mm in width) was defined as fold-line

57

due to the difference of flexibility between the printed and unprinted area

after baking. On the PEC collection tab, a paper auxiliary zone with a diameter of 10.0 mm was designed which could be properly and exactly aligned to the paper sample zone on PEC reaction tab after being folded along the predefined fold-line. Furthermore, the two rectangular paper legs, comprised of unprinted rectangular area (10.0 mm × 20.0 mm) and green wax-printed square area (10.0 mm × 10.0 mm) with a diagonal fold-line, were designed as the electrode substrates to integrate paper supercapacitor into the µ-PECOD. Detail procedures of wax-printing were described in Supporting Information. The unprinted area (paper auxiliary zone, paper sample zone and the electrode substrates for the supercapacitor) still maintained good hydrophilicity, flexibility, and porous structure and will not affect the further modifications

17

. In addition, the hydrophilic paper auxiliary zone and paper sample

zone constituted the reservoir of the paper PEC cell (~50 µL) after being folded at the predefined fold-line.


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Scheme 1. (A) The schematic representation, size, and shape of the wax patterns on this µ-PECOD. (B) One side of this µ-PECOD with the SPCCE and silver wire; (C) The reverse side of (B) with the SPCWE and silver wire. Then,

the

as-prepared

wax-penetrated

paper

sheets

were

ready

for

screen-printing of circuit components (Scheme 1B,C), including two carbon electrodes on its corresponding paper zone and silver wires, after cooling to room temperature (within 1 min). The electrode array consisted of a screen-printed carbon working electrode (SPCWE, 5 mm in diameter) on paper sample zone (Scheme 1B and Figure S2A) and screen-printed carbon counter electrode (SPCCE) on the reverse side of paper auxiliary zone (Scheme C and Figure S2B), respectively. After being folded along the fold-line, the two screen-printed electrodes will be connected once the paper PEC cell was filled with buffer solution. In addition, the paper sample zone on PEC reaction tab together with the SPCWE was denoted as “paper working electrode (PWE)” below (Scheme 1B,C). Finally, the silver wires were screen-printed from silver ink to fabricate the circuit for the connection of all components on this µ-PECOD (Scheme 1B,C, Figure S2C and Figure S2D). The carbon electrodes and silver wires could firmly attach to the paper surface due to the surface roughness of paper and binding reagents promoted transport or penetration of ink into the macroporous paper substrate 62. Integration of all-solid-state paper supercapacitor on µ-PECOD In this work, the all-solid-state paper supercapacitor was fabricated directly on the µ-PECOD through a simple two-step approach, that is, drawing of the thin film graphite electrodes 99 on the two paper legs by a graphite pencil, and the soakage and solidification of two paper legs in the H2SO4-PVA gel electrolyte. The greater



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suitability of the H2SO4-PVA gel electrolyte for flexible and all-solid-state energy storage devices has been well demonstrated 85. In a typical fabrication process, first, the two paper legs with graphite electrodes for the paper supercapacitor were fabricated through pencil drawing (manual mechanical abrasion) on the surface of two paper legs as indicated in Figure S2E and Figure S2F. This drawing was repeated for three times to form uniform coating on the surface of .two paper legs. Thereafter, the resulted paper sheet was cut into individual µ-PECOD by scissors. Subsequently, as shown in Figure 1A, the two paper legs were immersed into the hot solution of H2SO4-PVA gel electrolyte (the two green squares were kept out) for 10 min and picked out. The reason for the immersion in the hot solution was that, after it cooled down, the solution would become a little viscous, which would not be in favor of the thoroughly soakage of H2SO4-PVA gel electrolyte into the unprinted paper legs. Second, silver conductive adhesive was spread onto the green triangle (Figure 1Ba) followed by folding along the diagonal fold-line (Figure 1Bb), the silver wire could glue closely to the graphite coating with the aids of the silver conductive adhesive after folding. Then, the two paper legs were stacked quickly to form a structure of graphite-film/paper/paper/graphite-film as illustrated in Figure 1C followed by performing a even pressure of ~10 MPa (a weight of ~400 g) for 10 min to glue the two paper legs together through the adhesive polymer electrolyte. In addition, the two unprinted rectangular area of the paper legs soaked with H2SO4-PVA gel electrolyte functioned as both separator and electrolyte, which can minimize the


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total thickness and simplify the fabrication process. Finally, the all-solid-state paper supercapacitor was obtained after the evaporation of the excess water (Figure 1D). The typical thickness of the all-solid-state supercapacitor was ~0.50 mm. The scanning electron microscopy (SEM) images of this µ-PECOD were recorded on a JEOL JSM-5510 scanning electron microscope. Electrochemical characterizations were performed at room temperature using a CHI 660d workstation (CH Instruments Inc., Austin, TX).

Figure 1. (A) Immersing of the two paper legs of the µ-PECOD into H2SO4-PVA gel electrolyte; (B) Spreading silver conductive adhesive (a) onto the green triangle followed by folding along the diagonal fold-line (b); (C) Structure (cross section) of this paper supercapacitor of the µ-PECOD; (D) Photograph of this µ-PECOD. Construction of ssDNA1/CdS/PDDA-CNTs/PWE The PWE on the PEC reaction tab was firstly modified through sequentially



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assembling positively charged PDDA-functionalized CNTs (PDDA-CNTs) and negatively charged CdS NPs onto the surfaces of interwoven cellulose fibers in paper sample zone (Scheme S1A,B). Then, the ssDNA1 was immobilized into the CdS/PDDA-CNTs/PWE (Scheme S1C) through the classic EDC coupling reactions. Detail procedures were described in the Supporting Information. PEC operation and assay procedures of this µ-PECOD First, human serum sample solution (10 µL) contained different concentrations of ATP were mixed with phosphate buffer solution (PBS, pH 7.4, 10 µL) containing labeled ssDNA2 in a plastic vials. Then the mixed solution was added into the modified paper sample zone of PWE and allowed to hybridize for 180 s to form the ssDNA2/ATP/ssDNA1 complex (Scheme S1D) followed by washing thoroughly with PBS for preventing the nonspecific binding and achieving the best possible signal-to-background ratio. Thereafter, as shown in Figure S3A-C, the µ-PECOD was folded and clamped between two compatibly designed circuit boards in a model cassette (Figure S3Bd, demonstrated in our previous work

68

) to fix and connect this

origami device to the DMM (Direct current resolution: 0.01 µA; Frequency of measurement: 100K Hz, Figure S4). The detail circuit diagram of the entire system was shown in Figure S3D. The clamped µ-PECOD in the cassette was left free of external light through closing the black metallic cover (Figure S3Bb). Then, 0.1 M Tris-HCl buffer solution (pH 8.5, 50 µL) containing 2.0 mM H2O2 and 0.3 mM PIP was added into the paper PEC cell using a micro-syringe through the hole on the cassette (Figure S3Ba). As shown in Figure S3D, the generated photocurrent was


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collected between the modified PWE on reaction tab and the carbon counter electrode on collection tab to charge the paper supercapacitor for 60 s. A high instantaneous current through the DMM was obtained once the switch was closed (Figure S3D). Finally, the Max/Min button (Figure S4) on the DMM was pressed once to display the maximum value of measurements.

Result and Discussion Characterizations of ssDNA1/CdS/PDDA-CNTs/PWE Prior to the immobilization of CdS NPs into the paper sample zone of PWE, PDDA-CNTs was assembled onto the surfaces of interwoven cellulose fibers through electrostatic interactions to enhance the conductivity of paper sample zone and enlarge the effective surface area of PWE. The morphologies of PDDA-CNTs modified paper sample zones were characterized by SEM. Figure S5 showed that the nano-scale interconnected PDDA-CNTs were firmly entangled on micro-scale cellulose fibers in the form of small bundles or single tubes. To confirm the electrochemical properties of the resulted PDDA-CNTs/PWE, cyclic voltammetry (CV) was employed as a valuable tool to characterize the changes of the PWE behavior. Figure 2 showed the CVs of 10.0 mM [Fe(CN)6]3-/4- solution containing 0.5 M KCl in the PWE of different stages (Detail procedures of electrochemical characterization was described in Supporting Information). As can be seen in Figure 2, the bare PWE without modifications in paper sample zone exhibited one set of well defined redox peaks toward [Fe(CN)6]3-/4- (curve a). When the PWE was treated with PDDA-CNTs, there was a sharp increase of the peak current (curve b). The CV curves



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showed that the PDDA-CNTs/PWE had a significant higher peak current and larger CV area compared with the bare one, indicating that this PDDA-CNTs/PWE had a much larger effective surface area than bare one, which may be attributed to the macroporous morphology of paper and the fast electron transfer rate of the well interconnected CNTs on surfaces of interwoven cellulose fibers.

Figure 2. CVs of the PWE with different surface condition of the cellulose fibers: (a) bare PWE; (b) PDDA-CNTs/PWE; (c) CdS/PDDA-CNTs/PWE; (d) ssDNA1/CdS/PDDA-CNTs/PWE. Scan rate: 50 mV·s-1. The CVs of ssDNA1/CdS/PDDA-CNTs/PWE were also recorded in Figure 2. The assembly of CdS NPs into the PDDA-CNTs/PWE partially blocked the electron transfer of the redox probe and resulted in a decrease of CV (curve c). After the immobilization of ssDNA1 into the CdS/PDDA-CNTs/PWE, an obvious decrease in the peak current and an increase in the peak-to-peak separation were observed (curve d). The reason was that ssDNA1, acted as the inert electron- and mass- transfer blocking layer, hindered the diffusion of ferricyanide toward the surfaces of modified cellulose

fibers.

These

results

were

consistent

with

ssDNA1/CdS/PDDA-CNTs/PWE was fabricated as expected.


the

fact

that

the

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Figure 3. (A) Photocurrent action spectra of the PDDA-CNTs/PWE (a) with and (b) without CdS NPs. Inset: Absorbance spectrum of CdS NPs. (B) Photocurrent response of (a) CdS/PDDA-CNTs/PWE, (b) CdS/PWE, and (c) CdS/PDDA-CNTs/ITO under external light source; (C) Photocurrent response of ssDNA1/CdS/PDDA-CNTs in PWE (a-c) and on ITO (d) after addition of ssDNA2 in the presence of 0 pM (b), 10 pM (a, d), 20 pM (c) ATP with internal CL light source. PEC responses of CdS/PDDA-CNTs/PWE under external light source First of all, the PEC properties of the CdS/PDDA-CNTs/PWE were examined using 500 W Xe lamp (CHF-XM500W, Beijing Changtuo, China) equipped with monochromator as irradiation source (Detail procedures of this PEC characterization under external light was described in Supporting Information). The photocurrent generated by this CdS/PDDA-CNTs/PWE in the presence of H2O2 as a sacrificial electron donor was examined using a Tris-HCl buffer (0.1 M, pH 8.5) as electrolytic aqueous solution and measured on an electrochemical workstation at an applied potential of 0 V (vs. Ag/AgCl). Under illumination of external light (Scheme 2A), CdS NPs in paper sample zone were photo-excited to produce an electron (e-)-hole (h+) pair. The injection of the conduction-band (CB) electrons into the SPCWE through PDDA-CNTs yielded the photocurrent, whereas the electron donor (H2O2 in solution) provided the electrons to the valence-band (VB) holes to thus complete the photocurrent generation cycle. The resulting photocurrent action spectrum of CdS/PDDA-CNTs/PWE was depicted in Figure 3A, curve a. The photocurrent spectrum followed well the profile of the absorption spectrum of the CdS NPs (Insert



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in Figure 3A) which implied that the CdS NPs in the PDDA-CNTs/PWE did not aggregate during the assembling processes

73

. In a control experiment, no

photocurrent was observed upon irradiation of a PDDA-CNTs/PWE that lacked the CdS NPs (Figure 3A, curve b). These results indicated that the photocurrent originated from the CdS NPs and that the PDDA-CNTs were not photoactive in this process. According to Figure 3A, curve a, 417 nm was chosen as the external excitation wavelength in the following PEC experiments with external light source.


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Scheme 2. Schematic illustration of photocurrent generation mechanism in the modified paper sample zone of PWE under (A) external physical light source and (B) internal CL light source; (C) Storage of the generated photocurrent in paper supercapacitor for 60 s; (D) Instantaneous release of the stored electrical energy through the DMM once the switch was closed. In a further control experiment, CdS NPs were assembled on the surfaces of cellulose fibers in PWE functionalized with PDDA through the electrostatic interactions under the same experimental conditions in each case. Compared with the CdS/PDDA-CNTs/PWE (Figure 3B, curve a), the photocurrent generated by



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CdS/PWE that lacked the CNT connectors increased tardily and the photocurrent intensity was low (Figure 3B, curve b). Therefore, it could be conclude that the CNTs which bridge the CdS NPs to the SPCWE (Scheme 2A and Scheme S1B) may then further facilitate the photocurrent generation by trapping the conduction-band electrons, a process that resulted in charge separation and retardation of the recombination process. That is, the conductive CNTs modified interwoven cellulose fibers provided efficient and fast electron transfer paths for the conduction-band electrons to the SPCWE. To compare the PEC performance built on different substrates, a control PEC aptasensor was fabricated on indium tin oxide (ITO) electrode with the same area (6 mm in diameter) using an identical process. Surprisingly, the CdS/PDDA-CNTs/PWE exhibited superior PEC performance to its ITO counterpart as shown in Figure 3B. The photocurrent of the CdS/PDDA-CNTs/ITO (Figure 3B, curve c) was roughly 3-fold lower than that in PWE (Figure 3B, curve a), which was mainly attributed to the enhanced immobilization capacity for CdS NPs in the modified macroporous paper sample zone with 3D interwoven cellulose fibers network as well as the effective light transmission across the total thickness of paper sample zone which was filled with solution

100

, resulting the increased total number of exited CdS NPs in

whole PWE (Scheme 2A and Scheme S1B). In addition, compared with curve b and curve c in Figure 3B, the photocurrents generated from the CdS/PDDA-CNTs/PWE (Figure 3B, curve a) could increase violently under irradiation and recover rapidly under the dark. Besides the high electron transfer rate of CNTs connectors, the fast


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and stable photocurrent responses in CdS/PDDA-CNTs/PWE may be also attributed to the rapid transport of sacrificial electron donor (H2O2) in the 3D macroporous paper sample zone 41. PEC responses of CdS/PDDA-CNTs/PWE with internal CL light source For the CL exited PEC in PWE, ABEI-AuNPs-H2O2-PIP CL system with superior CL property 92 was employed as simple and low-cost internal excitation light source in this work (Scheme 2B). The absorption spectrum of CdS NPs was found to overlap nicely with the CL spectrum of ABEI-AuNP label, λCL = 424 nm (Figure S6). The PEC responses of this CdS/PDDA-CNTs/PWE immobilized with the ssDNA1/ATP/ssDNA2 sandwich complexes labeled with ABEI-AuNPs in the presence of H2O2 and PIP were shown in Figure 3C. After the addition of 2.0 mM H2O2 and 0.3 mM PIP at 20 s into the above ssDNA1/ATP/ssDNA2 modified paper sample zone of PWE, robust and stable photocurrent (Figure 3C, curve a) was generated under illumination of the internal CL emission from ABEI-AuNP labels (Scheme 2B) and could maintain for 180 s. The ssDNA1/ATP/ssDNA2 sandwich complexes (cATP = 10 pM, curve a), gave much-higher PEC responses than the nonspecific adsorption of labeled ssDNA2 without ATP (curve b); indicated low levels of nonspecific adsorption of labeled ssDNA2 in paper sample zone were observed in the absence of ATP. Furthermore, increased ATP concentration leaded to the increased ABEI-AuNPs loading and thus boosted the generation of CL emission for enhanced photocurrent responding (cATP = 20 pM, curve c). Furthermore, the photocurrent yield from the same aptasensor on ITO (curve d)



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with internal CL light source was also lower than that in PWE (curve a) and decreased gradually under the same condition in each case (cATP = 10 pM), which may be attributed mainly to the increased illumination area and the fast transport of H2O2 in the macroporous paper sample zone compared with that on the relatively uniform and even surface of the CdS/PDDA-CNTs/ITO. That is, the CL emission generated from the ABEI-AuNP labels embedded in the 3D space among the interwoven CdS/PDDA-CNTs/cellulose fibers could be fully utilized in all directions (Scheme 2B). Influencing factors on PEC response in the modified PWE were investigated in Supporting Information. Amplified PEC assay with paper supercapacitor Due to the long durability of the CL emission from ABEI-AuNP labels

92

, the

resulted photocurrents could maintain for 180 s (Figure 3C). Therefore, herein, the paper supercapacitor could be charged stably by the photocurrent generated from PWE for 60 s using the generated photocurrent (Scheme 2C). Then, the paper supercapacitor was short connected (Scheme 2D) through a low-cost and handheld DMM with the Max/Min function, which could record and display the maximum value of measurements after pressing the Max/Min button once. Thus, the instantaneous amplified current from the paper supercapacitor could be obtained.


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Figure 4. (A) The relationship between current response and concentration of ATP with (red) and without (blue) the amplification of paper supercapacitor; (B) Current amplification of this µ-PECOD with paper supercapacitor in the presence of 1 pM ATP under different charging time. Eleven measurements for each point. As shown in Figure 4A, an enhancement of about 13-fold higher in current was observed from paper supercapacitor under different amount of the ATP compared with that from modified PWE directly. Furthermore, the results showed that both the photocurrent generated directly from modified PWE (IPWE) and the amplified current released from paper supercapacitor (IPS; charge time: 60 s) had a fairly good linear relationship with the concentration of the ATP ranging from 1.0 pM to 1.0 nM. The regression equations were expressed as IPWE (nA) = 1.93 c (pM) + 7.59 (R = 0.9968) and IPS (nA) = 27.312 c (pM) + 102.187 (R = 0.9976) (c represents the concentration of ATP) with the same limit of detection (defined as 3σ, where σ is the standard deviation of eleven measurements of blank samples) of 0.2 pM ATP, which was mainly attributed to the concomitant amplification of the background photocurrent from the modified PWE. The sensitivity of this µ-PECOD with paper supercapacitor (27.312 nA·pM-1) as indicated in Figure 4A was higher (~13-fold) than that without the amplification through paper supercapacitor (1.93 nA·pM-1). In addition, the limit of detection is lower than or comparable to those of other reported aptasensors for



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ATP, such as fluorescence aptasensors (28 nM

101

, 0.5 µM

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102

, and 10 µM

103

),

electrochemical aptasensors (0.1 pM 104, 15 nM 105, and 2.3 pM 106), surface-enhanced raman scattering aptasensor (12.4 pM

107

), and PEC aptasensor (3.2 nM

108

). It is

notable that the analytical performance of this µ-PECOD was related to the charging time. As shown in Figure 4B, using 1.0 pM ATP as a model, the amplified current from the paper supercapacitor increased with the increasing of charging time (0-180 s). At 60 s, the amplified current signal was ~129 nA, which was high enough to be sensitively detected by the DMM (direct current measurement resolution: 10 nA). Therefore, considering the development of this method to high sample throughput, the charging time was selected at 60 s. The analytical feasibility and accuracy of this µ-PECOD was evaluated in real samples (here, five healthy and fresh clinical human serums from local hospital) and compared with the reference high performance liquid chromatography (HPLC) method as gold standard, the latter was carried out with parallel single-analyte test in the local hospital. These results gave the relative standard deviation (RSD) less than 5.0% and were in good agreement with that obtained using the reference HPLC method, indicated an acceptable accuracy of this method. To further confirm the accuracy of this µ-PECOD, standard addition experiments were performed through adding 20.0 pM ATP into each sample. The recoveries were between 97.3% and 105.6%. These results revealed the practicality of this µ-PECOD for determination of ATP in human serum. Furthermore, to adequately validate and confirm the applicability and sensitivity of this µ-PECOD from 1.0 pM to 1.0 nM, spiked serum


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samples were prepared through adding different amounts of ATP (1.0 pM to 1.0 nM) into the human serum sample. The results were shown in Table 2 and an acceptable recovery (97.3% - 102.7%) and RSD (less than 5.0%) data was obtained. Hence, the developed µ-PECOD provided a possible application for the detection of ATP in real samples.

Table 1. Assay results of clinical human serum samples using the proposed µ-PECOD and reference HPLC methods. Proposed µ-PECOD (pM) Sample

Detected (pM)a

RSD b

Added (pM) c

Found (pM)d

RSD

Recovery (%)e

Sample-1 Sample-2 Sample-3 Sample-4 Sample-5

137.4 165.8 181.3 186.2 150.8

3.4 3.8 4.5 4.6 4.8

20.0 20.0 20.0 20.0 20.0

21.1 19.6 19.8 19.5 20.8

3.1 3.0 3.2 4.2 3.8

105.6 98.1 98.5 97.3 103.8

Reference HPLC (pM)

RSD

136.5 164.7 182.7 184.6 152.2

3.7 3.4 4.1 3.9 4.3

a

[Detected] means the amount of ATP got according to the linear standard curve equations from eleven parallel detections. bThe RSD of measurements are calculated from eleven independent experiments. c[Added] means the values that we add into human serum sample. d[Found] means the values obtained by subtracting the [Detected] value in the unspiked serum sample from the [Detected] value in the spiked serum sample. eRecovery means the ratio of [Found]/[Added].

Table 2. Recovery of ATP in human serum sample Sample Sample-1 Sample-2 Sample-3 Sample-4 Sample-5 Sample-6 Sample-7

cATP (pM)

Added (pM)

Detected (pM)

Found (pM)a

RSD (%, n=11)

Recovery (%)

131.8

1 5 15 60 240 630 910

132.77 136.92 141.53 193.4 368.4 781.2 1059.1

0.97 5.12 9.73 61.6 236.6 649.4 927.3

3.3 3.8 4.1 3.6 4.3 4.4 3.6

97.5 102.4 97.3 102.7 98.6 101.5 101.9

a

[Found] means the values obtained by subtracting the intrinsical value of [ATP] in the serum sample (here 131.8 pM) from the [Detected] value in the spiked serum sample.



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Specificity, reproducibility, and stability of this µ-PECOD In order to verify the specificity of the µ-PECOD toward ATP, experiments were performed by using three other ATP analogues in human serum: CTP, GTP, and UTP. Figure 5 compared the current responses of the µ-PECOD after hybridization with CTP, GTP, and UTP solutions under the same experimental conditions and at the same concentration of 50.0 pM. Figure 5 showed that CTP, GTP, and UTP did not exhibit any great increase of current. In contrast to the hybridization of this µ-PECOD with 50.0 pM ATP, the current greatly increased. Similarly, a mixed sample (50.0 pM ATP coexisted with 100.0 pM CTP, 100.0 pM GTP, and 100.0 pM UTP) did not exhibit major signal change compared with that of ATP alone. This comparison essentially suggested that this µ-PECOD was highly selective and had quite an affinity toward ATP. The reproducibility of this µ-PECOD was investigated based on inter-assay precision between ten µ-PECODs (measurements of the same sample on ten different µ-PECODs prepared in different batches). The RSD for the parallel detection of 0 pM, 10.0 pM, and 50.0 pM ATP with ten µ-PECODs, respectively, was 3.58%, 3.72%, and 3.64%. To investigate the stability of this µ-PECOD, it was stored in a valve bag at 4 ℃ and measured at intervals of three days, no obvious change was observed after 4 weeks. These results indicated that this µ-PECOD had good reproducibility and was fairly robust in normal storage conditions and achieved sufficient stability and precision during manufacture, storage or long-distance transport to remote regions and developing countries.


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Figure 5. The selectivity of this µ-PECOD to ATP.

Conclusion In this approach, a truly low-cost, simple, portable, and disposable µ-PECOD based on CL excited PEC assay and integrated paper supercapacitor was presented. The entire µ-PECOD was fabricated on a single sheet of flat paper in batch based on the principle of origami and kirigami, no complicated treatment was involved in this fabrication procedure, providing a simple and facile route to fabricate this µ-PECOD in bulk. The construction of PEC assay in the 3D macroporous PWE with interwoven cellulose fibers in paper sample zone showed an enhanced PEC response under both external and internal light source compared with traditional flat electrode counterpart with relatively uniform and even surface. The photocurrent generated from the PWE could be detected sensitively by a DMM through the amplification of paper supercapacitor, providing a low-cost approach for detection and diagnosis in locations where it was difficult or impractical to access expensive electrochemical workstation or lock-in amplifier. Furthermore, although there were still several steps involved for the PEC assay on this µ-PECOD, the operation of the simple DMM and µ-PECOD was more understandable than that of the sophisticated electrochemical workstation or



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lock-in amplifier. In conclusion, with its low-cost, simplicity, portability, and sensitivity, this current amplification strategy as a powerful tool holds great promise for other constant current-output analysis, such as fuel cell analysis, on Lab-on-paper device through changing the paper PEC cell on this µ-PECOD to paper fuel cell, and we believe it could be taken in the direction of point-of-care diagnosis in our future work. Acknowledgements This work was financially supported by Natural Science Research Foundation of China (21175058, 21277058, 21207048); Natural Science Foundation of Shandong Province, China (ZR2012BZ002) and Technology Development Plan of Shandong Province, China (2011GGB01153).

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For Table of Contents Only


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169x61mm (300 x 300 DPI)



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175x109mm (300 x 300 DPI)


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Photoelectrochemical Lab-on-Paper Device Based on an Integrated

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