Electrochemistry Communications 11 (2009) 1936–1939

Contents lists available at ScienceDirect

Electrochemistry Communications

journal homepage: www.elsevier .com/locate /e lecom

Mussel-inspired fabrication of encoded polymer filmsfor electrochemical identification

Yun Zhang, Hua Wang *, Jinfang Nie, Hui Zhou, Guoli Shen *, Ruqin YuState Key Laboratory of Chemo/Biosensing and Chemometrics, College of Chemistry and Chemical Engineering, Hunan University, Changsha 410082, PR China

a r t i c l e i n f o a b s t r a c t

Article history:Received 2 July 2009Received in revised form 12 August 2009Accepted 12 August 2009Available online 15 August 2009

Keywords:Encoded materialsProduct protectionElectrochemical identificationMussel-inspired surface chemistryDopamine

1388-2481/$ - see front matter � 2009 Published bydoi:10.1016/j.elecom.2009.08.024

* Corresponding authors. Tel./fax: +86 731 882 135E-mail addresses: huawang@hnu.cn (H. Wang), gls

Encoded polymer films for electrochemical identification have been achieved by embedding differentsemiconductor nanocrystals into self-polymerized dopamine films. Such encoded polydopamine filmsbased on mussel-inspired surface chemistry show high adhesive ability and can be created on a widerange of inorganic and organic materials, including noble metals, oxides, ceramics, and synthetic poly-mers. By incorporating different predetermined levels of various redox nanomarkers, the use of multi-film system composed of multiple, sequenced polydopamine identification films could theoretically gen-erate nearly unlimited (>1012) distinct voltammetric signatures (electric codes).

� 2009 Published by Elsevier B.V.

1. Introduction

Encoded nanomaterials have received extensive recent atten-tion for both fundamental research and practical applications[1–8]. One unique and important application of such materials istheir use as covert tagging systems for product-tracking/identifica-tion/protection, aiming at combating the escalating global threat ofcounterfeit products. Various encoded nanomaterials, such asmetallic phosphate nanoparticles [6], quantum dots (QDs)-incor-porated beads [7], and composite [8] or striped [7] metallic nano-wires, have been synthesized and used as covert redox tags inelectrical identification systems. Such redox-encoded nanomateri-als commonly contain different predetermined levels of multiplemetal tracers that can be distinguished by stripping voltammetry.This leads to thousands of recognizable multipotential/currentintensities voltammetric signatures (electric codes). Electrochemi-cal identification techniques benefit from cost, robust, and simpli-fication of operation. In particular, the involved electrochemicalanalyzers with low-power demands are ease to be miniaturizedto develop potable analytical devices for meeting many of therequirements of filed-based authenticity testing. However, the pre-viously implemented physical adsorption-based immobilization ofthe nanomaterial tags [7] would surfer from unfavorable stabilityof the resulting identification films in addition to the very limitedclasses of substrates to be coded. Moreover, the available redox-

Elsevier B.V.

5 (H. Wang).hen@hnu.cn (G. Shen).

encoded nanomaterials offer a low coding capacity with a very lim-ited number (<104) of usable electric codes.

Mussels are promiscuous fouling organisms which can secreteadhesive proteins to attach to virtually all types of inorganic andorganic surfaces [9,10]. Inspired by the unique amino acid compo-sition of the adhesive proteins in mussels, namely the coexistenceof catechol and amine groups, Lee et al. [11] recently used dopa-mine that contains both functionalities to spontaneously form thinpolymer films onto practically 25 diverse materials, includingclassically adhesion-resistant materials such as poly(tetrafluoro-ethylene) (PTFE). They argued for a possible polymerization mech-anism that involves oxidation of the catechol to a quinone followedby polymerization in a manner reminiscent of melanin formation.Herein we present a method based on such mussel-inspired sur-face chemistry to prepare adhesive redox-encoded polydopaminefilms onto various commercial products made from diversifiedclasses of materials for electrochemical identification. We alsoshow that it is possible to achieve nearly unlimited electric codes(>1012) by using a multi-film identification system consisting ofmultiple, sequenced polydopamine encoded films.

2. Experimental

CdS and PbS semiconductor nanocrystals were prepared basedon a literature protocol [12]. Briefly, a 20-mL 0.54 wt.% sodium cit-rate solution and a 1-mL 40 mM CdCl2 or Pb(NO3)2 solution weremixed under stirring. The resultant mixture was adjusted to a pHof 9, into which a 1-mL 40 mM Na2S solution was dripped carefullyresulting in a yellow (CdS) or black (PbS) product solution.

Y. Zhang et al. / Electrochemistry Communications 11 (2009) 1936–1939 1937

The preparation of encoded polydopamine films was accom-plished according to the method reported by Lee et al. [11] exceptthat the prepared redox nanoparticles with predetermined levelswere added into the original dopamine solution for coding pur-poses. Various proportions of CdS and PbS nanocrystals were firstmixed; each of the mixtures was then added into a dopamine solu-tion in 10 mM Tris–HCl (pH 8.5) to form a specific coding solution.The final dopamine concentration was 2 mg mL�1. To create a mul-ti-film identification system, multiple coding solutions (each of20 lL) were successively cast on the selected substrate in anappropriate spacing and were left at 4 �C for 12 h. When the self-polymerization occurred at the substrate surface, the CdS andPbS nanoparticles contained in the coding solution were accord-ingly incorporated into the resultant thin polydopamine matrix.Thorough washing was performed to remove the unincorporatednanocrystals, and the obtained encoded films were dried by N2

gas. To determine the designed sequences, a 5-lL dopamine solu-tion without metal markers was applied on the upper left of thefirst encoded polydopamine film to create an additional ‘‘guider”film.

For the electrochemical detection, each of the prepared en-coded polydopamine films was treated with a 50-lL 1 M HNO3

solution for 10 min. The obtained resultant mixture was trans-ferred into an electrochemical cell containing 0.2 M acetate buffersupporting electrolyte solution (pH 5.2). Electrochemical measure-ment of the dissolved nanoparticles was performed using a tradi-tional three-electrode system consisting of a glassy carbonworking electrode (coated with a pre-plated mercury film [7]), asaturated calomel counter electrode (SCE), and a platinum foil ref-erence electrode. The deposition protocol involved a 1-min condi-tioning step at 0.6 V, followed by a 2-min deposition at �1.4 Vunder stirring. Subsequent square-wave stripping detection wasperformed after a 15-s rest period (without stirring) from �0.8to �0.4 V with a step potential of 4 mV, the 20-mV amplitude,and a frequency of 25 Hz.

3. Results and discussion

The encoded polymer films described in this study are based onthe incorporation of different predetermined levels of CdS and PbSsemiconductor nanocrystals (serving as models of metal markers)within polydopamine carrier films, in a manner analogous to theoptical beads of Han et al. [5] or electric beads of Wang et al. [7].All of the previous efforts for electrical authenticity testing [6–8]

Fig. 1. Schematic illustration of the electrochemical identification technique based oncoloured spheres (blue and red) represent CdS and PbS nanocrystals whose numbers arevia a voltammetric stripping measurement after the dissolution of metal markers withpotentials in the single-film system are used to define a code, e.g., (1:1), the synthetical apfilm system leads to a novel code, e.g., (1:2; 2:1). (For interpretation of the references to

have focused on a single-film identification system (Fig. 1a), inwhich both current intensities and relative intensity ratio at differ-ent potentials are used for coding purposes. For example, (1:1) is adistinct code. Similar to the optical coding based on multicolor/emission intensities [5], m potentials (corresponding to metalmarkers) and n current intensities (reflecting their predeterminedlevels) could theoretically generate (nm�1) unique codes. Becausethe voltammetric stripping reading method commonly allowssimultaneous measurements of up to 5 metal markers in a singlerun (with minimal peak overlapping) [13], Wang et al. [7] arguedfor a realistic electric single-film identification scheme that coulduse 5 potentials and 6 current intensities to yield ca. 7 775(65�1) usable electric codes.

To increase the number of usable codes, herein we initially di-rect at developing a multi-film system comprised of multiple, se-quenced encoded polydopamine films. In such a novelidentification system, the peak current intensities and relativeintensity ratio at different potentials obtained from different filmsare synthetically used for coding the same substrate. For example,in a two-film system (Fig. 1b), the association of code (1:2) andcode (2:1) obtained from film 1 and film 2, respectively, leads toa new (synthetical) code, (1:2; 2:1). Thus, the use of m potentials,n current intensities and f identification films could yield (nm�1)f

codes in theory. In this way, it is possible to obtain over4.7 � 1012 ((65�1)3) usable codes using a three-film system in con-junction with 5 potentials and 6 current intensities. To prevent theconfusion of the film sequences, an additional small polydopaminefilm (as a ‘‘guider”) on the upper left of the first identification film(Fig. 1b, under the arrow) was created to determine the designedfilm sequences.

A key question for this powerful identification system iswhether the additional nanoparticles would influence the poly-dopamine’s adhesive ability. Accordingly, we examined the feasi-bility of fabricating a two-film system onto a PTFE substrate as amodel of classically adhesion-resistant materials. Fig. 2a–d dis-plays such assessment for the polydopamine films prepared fromoriginal dopamine solutions without or with the additions of1CdS/1PbS and 1CdS/2PbS. The obtained three polydopamine filmshave similar brown circular shapes (with a diameter of ca.5 mm) (Fig. 2a and b) but with different voltammetric character-istics (Fig. 2c and d). The addition of metal markers yield twowell-defined voltommograms in each of which two sharp, sym-metric, and independent peaks are present at �0.68 (Cd) and�0.51 V (Pb), respectively (Fig. 2d), whereas only low background

potential/current intensity coding in (a) single-film and (b) two-film system. Theused to illustrate the current intensity levels. Electric code readout is accomplishedstrong acid. While both current intensities and relative intensity ratios at differentplication of various codes from multiple sequenced identification films in the multi-

colour in this figure legend, the reader is referred to the web version of this article.).

-0.9 -0.8 -0.7 -0.6 -0.5 -0.4 -0.3

Cur

rent

(µA

)

Potential (V vs. SCE)

c)

d)

e)

1

1

2

2

Cd Pb

2 µµA

35 µµA

µ

Fig. 2. Photographs of polydopamine films (brown circular shapes) prepared onPTFE (a) without or (b) with different predetermined levels of metal markers addedin original dopamine solutions to form a two-film system (1CdS/1PbS and 1CdS/2PbS

correspond to films 1 and 2, respectively). The scale bar is 2.5 mm. Square-wavestripping voltammograms (c), (d), and (e) obtained from films (a) and (b) and thecoding solution of 1CdS/1PbS (1) and 1CdS/2PbS (2) after the dissolution in acidsolution, respectively.

-1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0

Cur

rent

(µA

)

Cd Pb4 µA

a1

a2

b1

b2

c1

c2

-1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0

4 µA

Potential (V vs. SCE)

d1

d2

e1

e3e2

f2f3

f1

Cd Pb

µ

Fig. 3. Stripping voltammograms of various encoded polydopamine films preparedin the two-film systems on (a) gold slice, (b) glass slice, (c) stainless steel, and (d)polyethylene (PE) film, or prepared in the three-film systems on (e) copper supportand (f) ceramic tile. The predetermined concentrations ratios of CdS and PbSnanocrystals in original dopamine solutions are (a) 3:3 and 2:2, (b) 2:3 and 1:1, (c)1:1 and 2:1, (d) 2:3 and 2:2, (e) 3:2, 1:1 and 1:1, and (f) 1:1; 3:3 and 2:2,respectively.

1938 Y. Zhang et al. / Electrochemistry Communications 11 (2009) 1936–1939

signals are observed (Fig. 2c). The results indicate that the CdSand PbS nanocrystals added in the original dopamine solutionhave no impact on the adhesive ability of the resulting polydop-amine films.

The ability to control the proportions of multiple markers with-in carrier materials (and thus tune the signal intensities to createunique codes) has proven to be a vital effect upon the success ofa coding technique [5,7]. As also illustrated in Fig. 2, the peak cur-rent intensities of 1.3Cd/1Pb (Fig. 2d, curve 1) vs. 1.1Cd/1Pb (Fig. 2e,curve 1) and 1Cd/2.3Pb (Fig. 2d, curve 2) vs. 1Cd/2.2Pb (Fig. 2e, curve2) are obtained from the dissolved CdS and PbS nanoparticles afterand before their incorporation within the polydopamine matrix,respectively. Such peak current ratios reveal that the signals followclosely the predetermined levels of the corresponding redox nano-crystals, 1:1 and 1:2, respectively and that the loading ratio of themetal markers can be controlled.

By adjusting the predetermined ratios of CdS and PbS nanopar-ticles, we have actually prepared multifarious encoded polydop-amine films (in two- or three-film systems) with distinctvoltammetric fingerprints on various material substrates (relevantto packages or surfaces of commercial products), including noblemetal (gold), metals with native oxide surfaces (copper and stain-less steel), ceramics (glass and ceramic tile), and synthetic poly-

mers (polyethylene (PE)) (Fig. 3). Note that, the implementeddestructive releasing step involving a strong acid solution couldnot only dissolute the metal tags, but also destroy the substratesbeing coded to a certain extent (e.g., metal copper and ceramictile). Other methods, such as X-ray fluorescence [8], could provideconvenient nondestructive readout of these encoded polymerfilms.

The realization of effective authenticity tests requires highlyreproducible results. The precision and uniformity of the encodedpolymer films were examined by fabricating six different polydop-amine identification films on PTFE (data not shown). The resultingstripping voltammograms are highly reproducible, reflecting thereproducibility of the self-polymerization preparation and acid dis-solution and of the electrochemical measurements; relative stan-dard deviations (RSD) of 9.2% and 8.4% were estimated for thecorresponding Cd and Pb peaks, respectively.

From the practical point of view, we further investigated thedurability of the as-prepared encoded polydopamine films. It wasexperimentally found that these polymer identification films werestable under harsh conditions and were durable over prolongedperiods. For example, no obvious changes were observed in theirvoltammetric profiles when the identification films had beenheated for several hours in a temperature range of 30–70 �C, placedin water overnight, or followed desired periods at room tempera-ture as long as the encoded films or the substrates had not beendestroyed.

4. Conclusion

In summary, we have described the creation of a library of elec-trical codes by embedding multiple redox nanoparticles into poly-dopamine films at precisely controlled ratios. As desired forpractical identification applications, the as-prepared encoded poly-mer films based on mussel-inspired surface chemistry show highadhesive ability and can be fabricated on a broad range of materialsubstrates. The use of multi-film system consisting of multiple, se-quenced polydopamine identification films can readily allow fornearly unlimited codes. Such a new electrical identification tech-nique thus represents a powerful addition to the arsenal of prod-uct-tracking/identification/protection.

Y. Zhang et al. / Electrochemistry Communications 11 (2009) 1936–1939 1939

Acknowledgements

This work was supported by the National Natural Science Foun-dation of China (Nos. 20775020 and 90817101), and ‘‘973” Na-tional Basic Research Program of China (No. 2007CB310500).

References

[1] D.C. Pregibon, M. Toner, P.S. Doyle, Science 315 (2007) 1393.[2] L. Qin, M.J. Banholzer, J.E. Millstone, C.A. Mirkin, Nano Lett. 7 (2007) 3849.[3] S.O. Meade, M.Y. Chen, M.J. Sailor, G.M. Miskelly, Anal. Chem. 81 (2009) 2618.[4] S. Derveaux, B.G. Stubbe, C. Roelant, M. Leblans, B.G.D. Geest, J. Demeester,

S.C.D. Smedt, Anal. Chem. 80 (2008) 85.

[5] M.Y. Han, X.H. Gao, J.Z. Su, S.M. Nie, Nat. Biotechnol. 19 (2001) 631.[6] G.D. Liu, H. Wu, A. Dohnalkova, Y.H. Lin, Anal. Chem. 79 (2007) 5614.[7] J. Wang, G.D. Liu, G. Rivas, Anal. Chem. 75 (2003) 4667.[8] J. Wang, G.D. Liu, Anal. Chem. 78 (2006) 2461.[9] G.A. Young, D.J. Crisp, in: K.W. Allen (Ed.), Adhesion, Applied Surface, London,

1982.[10] M.J. Sever, J.T. Weisser, J. Monahan, S. Srinivsan, J.J. Wilker, Angew. Chem. Int.

Ed. 43 (2004) 448.[11] H. Lee, S.M. Dellatore, W.M. Miller, P.B. Messersmith, Science 318 (2007)

426.[12] M.A. Correa-Duarte, M. Giersig, L.M. Liz-Marzán, Chem. Phys. Lett. 286 (1998)

497.[13] J. Wang, Analytical Electrochemistry, second ed., Wiley-VCH, New York, 2000,

p. 75.