Electronic Supplementary Information · Electronic Supplementary Information Paper-based Photoelectrochemical Immunoassay for Low-cost and Multiplexed Point-of-care Testing Panpan

Electronic Supplementary Information

Paper-based Photoelectrochemical Immunoassay for Low-cost

and Multiplexed Point-of-care Testing

Panpan Wang, Lei Ge, Shenguang Ge, Jinghua Yu,* Mei Yan, Jiadong Huang

Reagents and apparatus

The All reagents were of analytical grade and directly used for the following

experiments as supplied. Ultra-pure water obtained from a Millipore water

purification system (resistivity ≥ 18.2 MΩcm) was used in all assays and solutions.

All mouse monoclonal α-fetoprotein (AFP), carcinoma antigen 153 (CA153),

carcinoma antigen 199 (CA199), and carcinoembryonic antigen (CEA) capture

antibodies (Ab1), unlabeled AFP, CA153, CA199, and CEA signal antibodies (Ab2),

and standard AFP, CA153, CA199, and CEA solutions were all obtained from

Linc-Bio Science Co. Ltd. (Shanghai, China). Blocking buffer for the residual

reactive sites on the Ab1 immobilized paper microzones was phosphate buffer

solution (PBS) containing 0.5% bovine serum albumin (BSA) and 0.5% casein.

Tween-20 (0.05%) was spiked into 0.01 M PBS (pH 7.4) as a washing buffer to

minimize unspecific adsorption. The clinical serum samples were from Shandong

Tumor Hospital. Whatman chromatography paper #1 was purchased from GE

Healthcare Worldwide and used with further adjustment of size (A4 size).

Electronic Supplementary Material (ESI) for Chemical CommunicationsThis journal is © The Royal Society of Chemistry 2013

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 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 Ab2 (Ab2-AuNPs-ABEI) were synthesized according to previous work

with modification 1. CdS NPs (5.5 nm) were prepared in water 2. 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

3.

Design and fabrication of this μ-PPECD

The shape for printing μ-PPECD was designed using Adobe illustrator CS4. As

shown in Scheme S1, the wax patterns of this μ-PPECD was comprised of a PEC

reaction disc (Scheme S1A,B: red-circle with a diameter of 40.0 mm) and a PEC

collection pad (Scheme S1C,D) containing a blue isosceles trapezoidal support

(length of the upper side: 12.0 mm, length of the under side: 40.0 mm, height: 17.0

mm) and a blue rectangle (40.0 mm×11.0 mm) with two rectangular paper legs (10.0

mm×30.0 mm). On the PEC reaction disc, there were eight unprinted paper sample

zones with the same diameter of 6.0 mm (Scheme S1A). On the PEC collection pad,


an unprinted paper auxiliary zone with a diameter of 6.0 mm was designed.

Furthermore, the two rectangular paper legs, comprised of unprinted rectangular area

(10.0 mm × 20.0 mm) and green square area (10.0 mm × 10.0 mm) with a diagonal

fold-line (1 mm in width), were designed as the electrode substrates to integrate PSC

into the μ-PPECD.

Scheme S1. The schematic representation, size and shape of this μ-PPECD. (A) PEC reaction disc; (B) The reverse side of (A); (C) PEC collection pad; (D) The reverse side of (C); (E, F) Pictures of this μ-PPECD after assembling.


Wax-printing in bulk on A4 paper sheets was performed using a wax printer

(Xerox Phaser 8560N color printer) set to the default parameters in a high-resolution

printing mode (Figure S1 and S2). Then the wax-printed paper sheet was baked at

130 for 150 s to let the printed wax melt and penetrate through the paper to form

the hydrophobic patterns (Figure S3 and S4). After the curing process, the unprinted

area still maintained good hydrophilicity, flexibility, and porous structure and will not

affect the further modifications 4.

The as-prepared wax-penetrated paper sheets were then ready for screen-printing

of circuit components (Scheme S1B,C,D) after cooling to room temperature (within 1

min), including carbon electrodes (Figure S5 and S8) on its corresponding paper zone

and silver wires (Figure S6 and S7). 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 5.


Figure S1. Wax-printed PEC reaction discs on a paper sheet (A4) before baking.


Figure S2. Wax-printed PEC collection pads on a paper sheet (A4) before baking.


Figure S3. Wax-printed PEC reaction discs on a paper sheet (A4) after baking.


Figure S4. Wax-printed PEC collection pads on a paper sheet (A4) after baking.


Figure S5. PEC reaction discs on a paper sheet (A4) after screen-printing of SPCWEs.


Figure S6. PEC collection pads on a paper sheet (A4) after screen-printing of silver wires.


Figure S7. PEC collection pads on a paper sheet (A4) after screen-printing of silver wires (The

reverse sides of Figure S6).


Figure S8. PEC collection pads on a paper sheet (A4) after screen-printing of SPCCE.


Integration of all-solid-state PSC on μ-PPECD

In a typical fabrication process, first, the two paper legs with graphite electrodes

for the PSC were fabricated through pencil drawing (manual mechanical abrasion) on

the surface of two paper legs as indicated in Figure S9 and Figure S10. 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 PEC collection pad by

scissors. Subsequently, the two unprinted 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 great suitability of the H2SO4-PVA gel electrolyte for

flexible and all-solid-state energy storage devices has been well demonstrated 3. 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.


Figure S9. PEC collection pads on a paper sheet (A4) after drawing of graphite electrodes.


Figure S10. PEC collection pads on a paper sheet (A4) after drawing of graphite electrodes

(Reverse side of Figure S9)..


Figure S11. (A) After immersing into the hot solution of H2SO4-PVA gel electrolyte, silver conductive adhesive (a) was spread onto the green triangle followed by folding along the diagonal fold-line (b); (B) Schematic structure (cross section) of this PSC on the μ-PPECD.

Then, silver conductive adhesive was spread onto the green triangles (Figure

S11Aa) followed by folding along the diagonal fold-line (Figure S11Ab), 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 S11B

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 total thickness and simplify the fabrication process. Finally, the

all-solid-state PSC was obtained after the evaporation of the excess water. The typical

thickness of the all-solid-state PSC was ~0.50 mm. The scanning electron microscopy

(SEM) images of the graphite electrodes were recorded on a QUANTA FEG-250


scanning electron microscope. Electrochemical characterizations were performed at

room temperature using a CHI 660d workstation (CH Instruments Inc., Austin, TX).

Figure S12 showed that the graphite layer formed a continuous and uniform

coating (Figure S12A), comprised of multilayer graphenes (Figure S12B) with a large

portion of edge structures (Figure S12C), on the rough surface of paper, which have

been proven to be beneficial for electrochemical capacitance 6.

Figure S12. SEM images of the resulted graphitic film electrode on paper under different magnification.

Figure S13 (A) GCD curves of this PSC; (B) Durability test of this PSC by measuring 100 charge-discharge cycles.


Construction of Ab1/CdS/PDDA-CNTs/PWE

Firstly, PDDA-CNTs were prepared according to the following procedure, as

reported previously with some modifications 7. CNTs were first sonicated with 3:1

(v/v) H2SO4/HNO3 for 4 h to obtain carboxylic group-functionalized CNTs. The

resulting dispersion was filtered and washed repeatedly with water until the pH was

about 7.0. Then 0.5 mg·mL-1 CNTs were dispersed into a 0.25% PDDA aqueous

solution containing 0.5 M NaCl and the resulting dispersion was sonicated for 30 min

to give a homogeneous black suspension. Residual PDDA was removed by

centrifugation, and the complex was rinsed with water. The collected complex was

redispersed in water with mild sonicating.

The PWEs on the PEC reaction disc were firstly modified through sequentially

assembling positively charged PDDA-CNTs and negative charged CdS NPs onto the

surfaces of interwoven cellulose fibers in paper sample zones. Briefly, 20 µL solution

of PDDA-CNTs and 20 µL solution of CdS NPs was alternately dropped into the bare

paper sample zone (Scheme S2A,B) and kept it for 10 min. After each dropping step,

the paper sample zone was rinsed thoroughly according to the method demonstrated

in our previous work 8. Second, the Ab1 was immobilized into the

CdS/PDDA-CNTs/PWEs (Scheme S2C) through the classic EDC coupling reactions.

Typically, a fresh prepared 15 µL solution of EDC (10 mg·mL-1) and of NHS (20

mg·mL-1) were added into the CdS/PDDA-CNTs/PWE to active the COOH groups on

thioglycolic acid-capped CdS NPs for 30 min followed by washing thoroughly with

water, then 20 µL of 20 µg·mL-1 Ab1 solution was dropped into the activated PWE


and the PEC reaction disc was then incubated at 4 for 4 h. Subsequently,

physically absorbed excess Ab1 were rinsed with phosphate buffer solution (PBS, pH

7.4). Finally, the Ab1/CdS/PDDA-CNTs/PWE was blocked by the blocking buffer for

0.5 h to cover the possible remaining active sites. Electrochemical impedance

spectroscopy (EIS) was performed on an IM6x electrochemical working station

(Zahner Co., Germany).

Scheme S2. Schematic representation of the fabrication procedures for Ab1/CdS/PDDA-CNTs/PWE: (A) bare PWE: (a) wax-penetrated paper, (b) SPCWE, (c) cellulose fibers in paper sample zone (unprinted macroporous paper); (B) Sequential assembling of PDDA-CNTs and CdS NPs onto surfaces of cellulose fibers in bare PWE; (C) After the immobilization of Ab1 on the surfaces of PDDA-CNTs/CdS modified cellulose fibers in PWE and the incubation with antigen and Ab2 to form Ab1/antigen/ Ab2 sandwich complexes.


Figure S14. SEM images of PDDA-CNTs entangled cellulose fibers in paper sample zone.

Characterization of Ab1/CdS/PDDA-CNTs/PWE

The morphologies of PDDA-CNTs modified paper sample zones were

characterized by SEM. Figure S14 shows that the nano-scale interconnected

PDDA-CNTs were firmly entangled on micro-scale cellulose fibers in the form of

small bundles or single tubes.

The formation of PDDA-CNTs/CdS/Ab1 in PWE was further confirmed by the

EIS measurements, which is an effective tool for characterizing the interface

properties of electrodes. EIS were carried out in a background solution of 5 mM

[Fe(CN)6]3-/4- at a bias potential of 170 mV (versus Ag/AgCl).

As shown in Figure S15, the bare PWE showed a relatively small

electron-transfer resistance, Ret (curve a). After PDDA-CNTs was formed on the

cellulose fiber surface in paper sample zone, a much smaller Ret was observed (curve

b) while the anchoring of CdS NPs showed a much larger Ret (curve c). The reason

may be that the PDDA-CNTs immobilized on the cellulose fiber surfaces played an


important role similar to a conducting wire, which provided a fast electron transfer

path to the SPCWE, while the assembly of CdS NPs into the PDDA-CNTs/PWE

partially blocked the electron transfer of the redox probe. Remarkable increase in the

Ret value was observed after both the immobilization of Ab1 (curve d) and blocking

with BSA (curve e) on the surfaces of PDDA-CNTs/CdS modified cellulose fibers in

PWE, indicating that the electron-transfer kinetics of the redox probe was slow down,

which testified the successful immobilization of Ab1 and BSA blocking.

Figure S15. EIS of the PWE under different condition in 10.0 mM [Fe(CN)6]

3-/4- solution containing 0.5 M KCl: (a) bare PWE; (b) PDDA-CNTs/PWE; (c) CdS/PDDA-CNTs/PWE; (d) Ab1/CdS/PDDA-CNTs/PWE; (e) Ab1/CdS/PDDA-CNTs/PWE after blocking.

Influencing factors on PEC response

For the optimizations below, the photocurrent generated from this

Ab2/antigen/Ab1/CdS/PDDA-CNTs/PWE with internal CL light source was measured

directly on the electrochemical workstation at an applied potential of 0 V (vs.

Ag/AgCl) and recorded as the average photocurrent between the measurements from

80 s to 240 s (Fig. 1).


The incubation process was performed at room temperature. The successful

development of the multiplex immunoassay required that the common incubation time

must be suitable for all analytes. All the PEC responses for increased with the

increasing incubation time and then leveled off, which indicated maximum formation

of Ab2/antigen/Ab1 sandwich immuno-complexes in the modified PWE. The optimal

total incubation time of 10.0 ng·mL-1 AFP, 10.0 U·mL-1 CA153, 10.0 U·mL-1 CA199,

and 10.0 ng·mL-1 CEA immuno-complexes was 250 s, 240 s, 260 s and 240 s,

respectively. Hence, considering the optimal analytical performance and development

of this method to high sample throughput, the hybridization time of 240 s was

selected. The incubation process in this macroporous PWE needed shorter time

compared with 40 min for the traditional electrochemical immunoassay on flat

working electrode 9. This was mainly due to the high surface-to-volume ratio,

incompact macroporous structure, and the small volume of the PWE (0.18 mm

thickness and 6 mm in diameter). Thus, the Ab2 and antigen should diffuse only short

transport distances to react with the Ab1 10. Furthermore, as the solution dried in paper

sample zone of PWE, the concentration of each reagent increased; this concentration

maybe further enhance the binding kinetics of Ab2/antigen/Ab1 sandwich

immuno-complexes 10. The short hybridization time could be favorable to high sample

throughput and rapid POC diagnosis.


Figure S16. Effects of H2O2 concentration on photocurrent response at 20.0 ng·mL-1 CEA concentration, where n = 11 for each point.

In this work, as shown in Scheme 1, H2O2 was not only used as the co-reactant in

the ABEI-AuNPs-H2O2-PIP CL system, but also as the electron donors to suppress

the corrosion (lattice dissolution) of CdS NPs under illumination as well as to

facilitate the generation of stable photocurrent. Thus, the generated photocurrent was

optimized using CEA as a model by changing the concentration of H2O2 (Figure S16)

in the range of 2.0 to 5.0 mM. When the concentration of H2O2 was lower than 3.5

mM, the photocurrent increased with increasing H2O2 concentration; when the

concentration of H2O2 was further elevated, the photocurrent would decrease with

increasing H2O2 concentration. This may be attributed to the oxidation of the CdS

NPs by excess H2O2 that yielded surface defects and traps 11. These resulted traps

could enhance the recombination of the generated electron-hole pairs in the CdS NPs,

thus inhibiting the electron injection to the PWE. Therefore, 3.5 mM H2O2 was used

in this work.


Scheme S3. The schematic representation of the light use-efficiency (A) in the 3D space among the interwoven cellulose fibers in macroporous paper sample zone of PWE and (B) on relatively uniform and even surface of the ITO.

Figure S17. Enlarged photograph of the screen and buttons on DMM.

PEC operation and assay procedures of this μ-PPECD

First, sample solution (10 µL) containing different concentrations of AFP,

CA153, CA125, CEA were added to the corresponding modified paper sample zones

of PWE and allowed to incubate for 240 s at room temperature, followed by washing


with washing buffer. Then the labeled Ab2 PBS solution (10 µL, 20 µg·mL-1) was

added to corresponding PWEs, and allowed to incubate for 240 s at room temperature

to form the Ab2/antigen/Ab1 complex (Scheme S2C) followed by washing thoroughly

with PBS for preventing the nonspecific binding and achieving the best possible

signal-to-background ratio. Thereafter, as shown in Scheme S1E,F and Figure S18A,

the PEC reaction disc was reversibly installed onto the PEC collection pad by piercing

through the pivot points (indicated in Scheme S1) on the PEC reaction disc and PEC

collection pad using a thumbtack. Thus, the eight paper sample zones on the PEC

reaction disc could exactly and successively align to the paper auxiliary zone on PEC

collection pad through the rotation of the PEC reaction disc around the thumbtack to

lap the two green marked lines (indicated in Scheme S1) on the PEC reaction disc and

PEC collection pad, respectively. Then, the μ-PPECD was put into a model cassette

(demonstrated in our previous work 12) and clamped between two compatibly

designed circuit boards (Figure S18B) to fix and connect this μ-PPECD to the DMM.

The hydrophilic paper auxiliary zone and paper sample zones constituted the

reservoirs of the paper PEC cells (~50 μL) after being clamped between the two

compatibly designed circuit boards. The two screen-printed electrodes (SPCWE and

SPCCE) will be connected once the paper PEC cell was filled with buffer solution.

The detail circuit diagram of the entire system was shown in Figure S18C. For

CL excited PEC assay, 0.1 M Tris-HCl buffer solution (pH 8.5, 50 µL) containing 3.5

mM H2O2 and 0.5 mM PIP was added into the paper PEC cell using a micro-syringe

through the hole on the cassette. As shown in Figure S18C, the generated


photocurrent was collected between the modified PWE on reaction tab and the carbon

counter electrode on collection tab to charge the PSC for 60 s. A high instantaneous

current through the DMM was obtained once the switch was closed. Finally, the

Max/Min button (Figure S17) on the DMM was pressed once to display the maximum

value of measurements.

Figure S18. Operation and PEC assay procedures of this μ-PPECD. (A) Configuration of this entire system: (a) the two compatibly designed circuit boards, (b) copper conductor for the connection between alligator clip and silver wire, (c) alligator clip, (d) micro-syringe, (e) DMM, (f) switch, (g) cassette, (h) black metallic cover. The centre hole on the circuit board was used for the addition of H2O2-PIP into PWE. (B) After clamping the μ-PPECD between the two circuit boards and connecting with the DMM. (C)Detail circuit diagram of the entire system.


Figure S19. The relationship between current response and concentration of AFP with (red) and without (blue) the amplification of PSC.

As shown in Figure S19, the results showed that both the photocurrent generated

directly from modified PWE (IPWE) and the amplified current released from PSC (IPSC;

charge time: 60 s) had a fairly good logarithmical relationship with the concentration

of the AFP ranging from 0.01 ng·mL-1 to 65 ng·mL-1. The regression equation was

expressed as IPWE (nA) = 423.64lg[AFP(ng·mL-1)] + 861.91 (R = 0.9962) and IPSC

(nA) = 6353.83lg[AFP(ng·mL-1)] + 12931.3 (R = 0.9970) with the same limit of

detection of 2.3 pg·mL-1 (3σ).


Figure S20. The relationship between current response and concentration of CA153 with (red) and without (blue) the amplification of PSC.




of the CA153 ranging from 0.02 U·mL-1 to 70 U·mL-1. The regression equation was

expressed as IPWE (nA) = 479.35lg[CA153 (U·mL-1)] + 816.26 (R = 0.9965) and IPSC

(nA) = 7190.78lg[CA153 (U·mL-1)] + 12246.09 (R = 0.9971) with the same limit of

detection of 7.1 mU·mL-1 (3σ).


Figure S21. The relationship between current response and concentration of CA199 with (red) and without (blue) the amplification of PSC.




of the CA199 ranging from 0.05 U·mL-1 to 80 U·mL-1. The regression equation was

expressed as IPWE (nA) = 566.16lg[CA199 (U·mL-1)] + 748.48 (R = 0.9961) and IPSC

(nA) = 8491.26lg[CA199 (U·mL-1)] + 11224.21 (R = 0.9974) with the same limit of

detection of 16.3 mU·mL-1 (3σ).


Assay results of real human serum

Table S1. Assay results of real human serum by the proposed and reference method

AFP concentration (ng·mL-1) CA153 concentration (U·mL-1) Samples

Proposed method

Reference method

Relativeerror (%)

Proposed method

Reference method

Relative error (%)

Sample-1 9.8 10.3 -5.1 10.2 9.8 3.9

Sample-2 56.5 54.7 3.1 11.9 11.4 4.3

Sample-3 60.2 57.5 4.5 16.4 15.8 3.7

Sample-4 18.5 19.3 -4.3 86.1 90.4 -5.0

CA199 concentration (U·mL-1) CEA concentration (ng·mL-1) Samples

Proposed method

Reference method

Relativeerror (%)

Proposed method

Reference method

Relative error (%)

Sample-1 75.3 79.0 -5.0 30.7 31.9 -3.9

Sample-2 20.6 21.5 -4.4 48.4 51.1 -5.6

Sample-3 53.2 51.7 2.8 53.4 51.5 3.6

Sample-4 31.3 29.5 5.8 55.7 58.6 -5.2

According to the criteria suggested by Li 13, the cut-off values of the four tumor markers

(AFP, CA153, CA199 and CEA) in human serum are 25 ng·mL-1, 35 U·mL-1, 35 U·mL-1, and 5.0

ng·mL-1, respectively. Therefore, the blood donor of sample-1 may be at the highest intestinal

cancer and pancreatic cancer risk; the blood donor of sample-2 may be placed in the class of the

highest ovarian cancer risk; the blood donor of sample-3 can be placed in the class of the highest

cholangiocarcinoma risk; and the blood donor of sample-4 can be at the highest breast cancer risk.

Specificity of this μ-PPECD

Specificity of this μ-PPECD was evaluated by comparing the current responses of a specific

analyte in the absence and presence of three other non-cognate analytes as interferences. When the

concentrations of the interferents changed in the range of 50∼200 ng·mL-1 or 50∼200 U·mL-1, the

maximum changes of current responses for 5.0 ng·mL-1 AFP, 5.0 U·mL-1 CA153, 5.0 U·mL-1


CA199, and 5.0 ng·mL-1 CEA were 2.1, 1.7, 1.9, and 2.4%, respectively, which essentially

indicated that the cross reactivity between these antibodies and their non-cognate antigens in the

PWE was negligible.

Reproducibility and stability of this μ-PPECD

The reproducibility of this μ-PPECD for CEA (as a model) was investigated with inter-assay

precision. The RSD for the parallel detections of 0 ng·mL-1, 1.0 ng·mL-1, and 25.0 ng·mL-1 CEA

with ten μ-PPECDs was 2.87%, 3.24%, and 2.76%, respectively. When this μ-PPECD was stored

in a valve bag at 4 °C and measured at intervals of three days, no obvious change was observed

after 4 weeks under ambient conditions. These results indicated that this μ-PPECD has better

reproducibility, stability, and precision during manufacture, storage or long-distance transport to

remote regions and developing countries.

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Electronic Supplementary Information · Electronic Supplementary Information Paper-based Photoelectrochemical Immunoassay for Low-cost and Multiplexed Point-of-care Testing Panpan