Nanocomposites of Gold Nanoparticles@Molecularly Imprinted

Review pubs.acs.org/cm

Nanocomposites of Gold Nanoparticles@Molecularly Imprinted Polymers: Chemistry, Processing, and Applications in Sensors Randa Ahmad,† Nébéwia Griffete,‡ Aazdine Lamouri,† Nordin Felidj,† Mohamed M. Chehimi,§ and Claire Mangeney*,† †

Université Paris Diderot, Sorbonne Paris Cité, ITODYS, UMR 7086 CNRS, 15 rue J-A de Baïf, 75205 Paris, Cedex 13, France Université Pierre et Marie Curie, UMR 8234 UPMC-CNRS, 4 place Jussieu, 75 252 Paris Cedex 05, France § Université Paris-Est, ICMPE UMR CNRS 7182, UPEC, 2-8 rue Henri Dunant, 94320 Thiais, France ‡

ABSTRACT: Gold nanoparticles (AuNPs) have stimulated a wide range of interest these past years due to their remarkable optical, electronic, and catalytic properties. Generally, the use of these nanoparticles requires their functionalization or combination with functional molecules, the nature of which depends on the target application. Among the numerous possibilities offered by chemistry, some recent papers report the coupling of AuNPs with molecularly imprinted polymers (MIPs) for the design of plasmonic-based AuNPs@MIP sensors. In such systems, a target analyte can be captured from a complex medium with a high specificity and selectivity owing to the exceptional chemical properties of the MIP matrix while the recognition event can be translated into a measurable physical signal (optical, electric, piezoelectric), the enhancement of which can be mediated by AuNPs. Despite such unique and intriguing advantages of AuNPs@MIP nanocomposites, there are still only limited numbers of studies regarding this field at the interface between plasmonics and functional polymers. This review focuses on the chemistry, processing, and applications of these nanohybrid materials, especially in the field of highly sensitive sensors. A prospect for the exploration of novel multicomposites combining AuNPs@MIPs with other kinds of nanoparticles (such as carbon nanotubes, graphene, and TiO2) is provided along with original strategies to optimize the functionality and sensitivity of these nanocomposites-based sensors.

I. INTRODUCTION I.1. State of the Art. Gold nanoparticles (AuNPs) have been the subject of extensive research during the past decade due to their outstanding properties and potential applications in many fields, such as biology, catalysis, electronics, and plasmonics.1 AuNPs with the diameter of 1−100 nm have large surface-tovolume ratio and high surface energy providing a stable immobilization platform for (bio)molecules, while preserving their reactivity.2,3 From an optical point of view, the dependence of their localized surface plasmon resonance (LSPR) on the dielectric environment provides the basis for currently developed applications of gold nanoparticles as elementary optical sensors transforming specific interactions between probe molecules attached to the sensor and target molecules into a detectable optical signal. Furthermore, excitation of LSPR modes by an external electromagnetic field results in a strong increase in the intensity of the local electric field in the vicinity of gold nanoparticles, by several orders of magnitude, which is widely used for Surface enhanced Raman spectroscopy (SERS). Such particles have also the capacity to allow fast and direct electron transfer between a broad series of molecules and electrodes, which makes them attractive for amplifying the performances of electrochemical sensors. However, the synthesis of gold nanoparticles is often not sufficient to make them useful for © 2015 American Chemical Society

most applications. Indeed, it is imperative to modify the nanoparticle surface with the proper organic or inorganic material to ensure stability under the necessary conditions as well as to provide added functionality for specific recognition properties. Among the recognition systems used in the literature for implementation in (bio)sensors, molecularly imprinted polymers have stimulated a wide range of interest over these last years because they are synthetic tailor-made receptors capable of selectively recognizing and binding target molecules with high affinity.4−7 Compared to other recognition systems, molecularly imprinted polymers (MIPs) possess many figures of merit, such as low cost and easy synthesis, high stability to harsh chemical and physical conditions, and excellent reusability. In molecular imprinting techniques, functional monomers are first preorganized in the vicinity of a multivalent analyte, copolymerized with cross-linking monomers, and after removal of the analyte, the polymer contains complementary binding sites for rebinding the analyte with high specificity. An important challenge for the development of sensors based on MIPs is to succeed in Received: January 13, 2015 Revised: May 8, 2015 Published: May 11, 2015 5464

DOI: 10.1021/acs.chemmater.5b00138 Chem. Mater. 2015, 27, 5464−5478

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Chemistry of Materials Table 1. Titles, Scopes, and References of the Main Reviews Focusing on the Topics of MIPs and AuNPs

date of publication

ref

review on MIPs from the standpoint of materials science

2008

4

general overview on MIPs in biosensing applications and description of the photopolymerization and photopatterning of MIPs description of the main approaches and challenges in the field of macromolecularly imprinted polymers review on biosensors including metal nanoparticles and plastic antibodies recent advances in construction of AuNPs-based biosensors advantages of nanomaterials (AuNPs, quantum dots, polymer NPs, carbon nanotubes, nanodiamonds, graphene) for biosensing

2012

5

2011

7

2014 2010 2014

8 3 2

running title Molecular Imprinting: Synthetic Materials as Substitutes for Biological Antibodies and Receptors Photopolymerization and Photostructuring of Molecularly Imprinted Polymers for Sensor Applications Critical Review and Perspective of Macromolecularly Imprinted Polymers Advancements in Nanosensors Using Plastic Antibodies Gold nanoparticle-based biosensors Nanomaterials for Biosensing Applications: a review

scope of the review

• Electronic properties (enhancing the electron transfer between electrodes and a large number of molecules) and catalytic properties.1 II.1. AuNPs-Based Optical Sensors. The optical properties of metallic nanoparticles (mainly gold, silver, copper) are due to the excitation of localized surface plasmon (LSP) modes at the particle surface (see illustration Figure 1a).12 The LSP modes,

transducing the recognition event into an analytical signal.8 This could be achieved by elaborating hybrid systems coupling MIPs with NPs in order to provide new forms of electronic or optical transduction. The resulting nanocomposites offer enhanced chemical and physical characteristics such as high specific area, superior mechanical properties, electrical conductivity, or optical properties, thereby improving their sensitivity and specificity.9 I.2. Scope of the Review. Many comprehensive surveys of scientific papers on the topic of molecular imprinting4,5,10 on the one hand and on gold nanoparticles2,8,11,12 on the other hand have already been reported in the literrature (see references with their corresponding scope in Table 1). Nevertheless, the interface between these two research fields combining AuNPs and MIPs in a single nanocomposite has never been the main focus of a review so far. In the present review, we consider the combination of molecularly imprinted polymers and gold nanoparticles from the standpoints of materials science in general and interface chemistry in particular, examining the most recent developments in the area. The first part of this review will summarize the fundamentals of molecular imprinted polymers and plasmonic nanostructures, followed by a description of representative new matrices that have been combined with AuNPs in a single nanocomposite. Thin nanocomposite films and colloidal particles of AuNPs@MIPs are covered in separate sections, because they represent different areas of increased research focus. As an important step toward new high performance materials with added-value properties, the final section is devoted to the elaboration of multicomposite materials from AuNPs@MIPs combined with other kinds of nanoparticles (carbon nanotubes, graphen, Pt, or TiO2). We will finish with concluding remarks and a few perspectives.

Figure 1. (a) Surface plasmon resonance excitation on a spherical NP; (b) UV−vis−NIR spectra of AuNPs with varying shapes and sizes. Reprinted with permission from ref 12. Copyright 2015 American Chemical Society.

II. PHYSICOCHEMICAL PROPERTIES OF METALLIC NANOPARTICLES There are several noble metal nanoparticles employed for sensor devices and biomedical applications. However, gold nanoparticles are mostly used owing to various appealing characteristics such as • Biocompatibility13 and chemical inertness; • Relatively simple production and modification1 through thiol linkages; • High surface energy providing a stable immobilization platform for biomolecules;3 • Remarkable optical properties due to the collective oscillation of the material’s conductive electrons (plasmons) induced by the interaction of an electromagnetic field with the metallic particles;

generally excited by electromagnetic waves ranging from the UV to near-infrared spectral range, are the result of collective oscillations of the conduction band electrons at the particle surface. The excitation of LSPs leads to the emergence of enhanced extinction (absorption and scattering) bands in the visible and near-infrared range, as shown in Figure 1b. The LSP modes strongly depend on the nanaparticle’s size and shape, the chemical nature of the metal, and the local surrounding medium. This environmental dependency of the LSPR represents a great advantage for analytics since the recognition event can result in a detectable change of its frequency.13 Moreover, a giant amplification of the local electromagnetic field emerges at the particle surface. As a consequence, when molecules are adsorbed on the AuNPs surface, a surface plasmon assisted signal amplification of the vibrational spectrum is obtained, moving the detection limits down to the single molecule level.14 5465


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chemistry and are polymerized in its presence. A cross-linked polymer network is formed, containing the template molecules. Then, the imprint molecules are removed from the polymer matrix. This leaves behind some “memory sites”, corresponding to binding sites specific to the template molecule of interest (Figure 2). These memory sites formed in the polymer are complementary, both sterically and chemically, to the template molecule. The polymer matrix is thus able to selectively rebind the imprinted molecule from a mixture of closely related compounds. The nature of the interaction between the template molecule and the MIPs differs depending on the approach used for the imprinting process. The first approach, proposed by Wulff and Sarhan in 1972,16 is based on a covalent coupling between the monomers and the template prior to polymerization. In contrast, the approach developed by Mosbach and co-workers4 consists in noncovalent interactions, the functional monomers forming a complex with the target molecules. The strategy based on weak noncovalent bonds is the most widely used method for the elaboration of MIPs as it offers a large variety of functional monomers for a broad spectrum of template molecules. III.2. Nanostructuration of MIPs. The major drawback of traditional imprinting methods is the obtention of MIP particles with high polydispersity and irregular sizes (from a few nanometers to hundreds of micrometers). When the particle size is large, the binding sites are deeply burried within the polymer matrix and poorly accessible to the template molecule. This leads to a low binding capacity and slow binding kinetics. To overcome these difficulties, nanoscale MIPs (NPs, nanowires, or nanotubes) have been prepared.6,17 These nanostructured imprinted materials present high surface-to-volume ratios, enhancing both the accessibility of the analyte to the recognition sites and the corresponding kinetics.18 Hybrid nanomaterials or nanocomposites were also designed by grafting thin layers of MIPs on inorganic nanoparticles such as carbon nanotubes (CNTs), graphene (GPH), quantum dots (QDs), gold NPs (AuNPs),19 or magnetic NPs (MNPs). These nanocomposites retain the specific physical properties of the core and the recognition capacity of the shell giving rise to MIPs with electrical, optical, or magnetic properties, which is interesting for the development of analytical assays. This review describes recent examples, focusing specifically on the combination of gold nanoparticles and MIPs for the development of sensitive and specific sensors. This includes an extensive review of the topic, by distinguishing the nanocomposites of AuNPs@MIPs in the form of (i) thin composite films or (ii) colloidal composite particles.

Such remarkable optical properties have stimulated many researches on the use of AuNPs for (bio)sensing, second harmonic generation, or surface enhanced spectroscopies (SERS, surface enhanced fluorescence). Gold nanoparticles have also demonstrated their advantages in (bio)analysis using the wellknown surface plasmon resonance (SPR) transduction. The SPR method is usually based on the change of the dielectric constant of propagating surface plasmons’ environment at the interface gold film/dielectric where the detection of the analyte can be recorded in different ways like the changes of the angle, intensity, or phase of the reflected light.13 Besides the use of a pure gold nanoparticle based SPR transduction replacing the gold film, a significant enhancement of the SPR signal can be obtained when gold films and gold nanoparticles are used in a sandwich configuration. In fact, the excitation of the LSP mode on gold nanoparticles provokes a perturbation of the evanescent field at the gold film/dielectric interface, improving the detection of the interaction between the immobilized (bio)receptor unit and the recognized analyte. II.2. AuNPs-Based Electrochemical Sensors. Gold nanoparticles present also conductivity and catalytic properties which are appealing to optimize the sensitivity of electrochemical sensors by amplifying the electrode surface, enhancing the electron transfer between electroactive species and the electrode, and catalyzing electrochemical reactions. For example, the conductivity properties of AuNPs were shown to enhance the electron transfer between the active centers of proteins and electrodes and thus to act as electron transfer “electron wires”.3 Furthermore, while bulk gold is chemically inert, AuNPs could also act as electrocatalysts,1 which was widely exploited in electrochemical biosensing to decrease overpotentials of some electrochemical reactions, favor the reaction reversibility, and design enzyme-free biosensors. II.3. AuNPs-Based Gravimetric Sensors. Piezoelectric sensors could also incorporate AuNPs in order to improve their sensitivity and detection limit. The sensitivity of such sensors, the most common of which is the quartz crystal microbalance (QCM), has to be improved in order to detect trace amounts of target molecules. As the high density and large surface-to-volume ratio of AuNPs can amplify the mass change on the crystals during the analysis, numerous research groups focused on improving the analytical sensitivity by coupling AuNPs with the QCM sensing process.15 Table 2 summarizes the various kinds of AuNPs-based sensors, highlighting the addedvalue properties brought by the nanoparticles. Table 2. AuNPs-Based Sensors sensor properties

added-value properties brought by the AuNPs

optical sensors based on changes in optical properties electrochemical sensors based on changes in electrical characteristics gravimetric sensors based on changes in mass

amplification of refractive index changes increase in electron transfer; improved surface area; catalysis enhancement of mass changes; improved surface area

IV. GENERAL METHODS FOR THE PREPARATION OF AUNPS@MIPS The integration of AuNPs into the MIP matrix could be performed following two approaches: (i) the immobilization of prefabricated AuNPs within the nanocomposite through chemical interactions and (ii) the in situ synthesis from the metal ion precursor previously chelated in the polymer matrix, by reducing it directly within the polymer chains. Examples of these two approaches are summarized in Table 3. The first approach involves first synthesizing colloidal particles in solution and subsequently incorporating them into the MIP matrix. Many ways were proposed based on this approach: • The step-by-step immobilization of AuNPs and MIP layers on surfaces. In this case, the AuNPs are not uniformly distributed along the thin polymer layer but form a

ref 13 1 15

III. FUNDAMENTAL ASPECTS OF MOLECULARLY IMPRINTED POLYMERS (MIPS) III.1. Principle. Molecularly imprinted polymers are crosslinked polymer networks containing specific recognition sites for a desired template molecule.4,5 The functional monomers interact with the template molecule via covalent or noncovalent 5466


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Figure 2. Scheme of the elaboration of MIPs. (a) Functional monomers interact first with the template molecule and are then polymerized, in its presence; (b) the template is subsequently removed, leaving behind the “memory sites” or binding nanocavities.

Table 3. General Methods for the Preparation of AuNPs@MIPs

the conventional approach of separately synthesizing AuNPs and MIPs, the in situ synthesis of AuNPs within MIPs remains far less explored. Nevertheless, this approach offers several advantages, including the possibility for generating smaller particles, stabilization of the NPs through surface attachment, and decreased aggregation due to their immobilization on the polymer chains. Yet, the NPs are not strongly attached to the MIPs. They remain within the polymer matrix if the polymer netwok is tight enough to prevent them from relocalization, redistribution, or leakage from the hybrid microgel.24 From the materials perspective and considering the survey of the literature on AuNPs@MIPs, one could distinguish two main kinds of composites of MIPs with AuNPs: (1) nanocomposite thin films and (2) colloidal nanocomposites (see Figure 3). In this review, these two types of AuNPs@MIPs materials will be described and illustrated separately.

monolayer of particles connected to the substrate and covered by the MIP film.20 • The electropolymerization of AuNPs functionalized by monomers. The main interest of this approach relies on the fact that the particles are trapped within the growing polymer instead of being confined to the surface as in the previous case. This technique yields a three-dimensional distribution of AuNPs within the MIP matrix and facilitates charge shuttling throughout the whole nanocomposite material.21 • The simple entrapment of AuNPs chemically protected by polar and lipophilic groups within the MIP network during its synthesis. The major drawback of this approach is that the AuNPs are not chemically connected to the MIP matrix and could leak from the nanocomposite in its fully swollen state. Nevertheless, if the polymer network is narrow enough to immobilize the AuNPs, their distribution was shown to remain constant within the matrix.22 • The synthesis of individual core−shell AuNPs@MIPs particles by grafting the MIP layers from the surface of the gold cores. This approach allows a fine control of the interface between the AuNPs and the MIP layers, but it requires some sophisticated functionalization steps to graft the polymer layers to the NPs surface without disturbing their colloidal stability.23 In contrast, the in situ approach is based on the reduction of gold ions adsorbed onto the polymer chains. In comparison to

V. NANOCOMPOSITE THIN FILMS When AuNPs@MIPs composites are prepared in the form of thin films, they can be implemented in optical, gravimetric, or electrochemical devices for sensing applications. The advantages of these sensors are their ease of preparation using spin-cast powders or in situ prepared films, their low production costs, and their very low detection limits. According to the type of transduction mechanism, AuNPs-based sensors were examined separately in this review. 5467


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aminothiophenol-modified electrodes for the amperometric detection of tolazoline.33 In another approach, sensitive films for the recognition of acetylsalicylic acid (aspirin) were elaborated by copolymerization of p-aminothiophenol and HAuCl4 on the Au electrode surface.34 However, presynthesized AuNPs functionalized by aniline moieties could also be used as comonomers with p-aminobenzenethiol for detection of trace dopamine35 with a detection limit of 7.8 nmol L−1 or directly as monomers for obtaining oligoaniline cross-linked AuNPs films polymerized at the Au electrode for sensing TNT (see Figure 5).

Figure 3. Schematic representation of (a) nanocomposite thin films and (b) colloidal nanocomposites.

V.1. AuNPs@MIPs Nanocomposite Thin Films for Electrochemical Sensors. Electrochemical sensors convert a binding event into electrical signals and offer fast, simple, and low-cost detection capacities. The introduction of gold NPs in MIPs has been reported to increase the electrode specific area, enhance the electron transfer between the recognition sites and the electrochemical transducer, and act as catalysts to amplify electrochemical reactions. Depending on the chemical nature of the AuNPs@MIPs nanocomposite film, different types of electrochemical sensors can be defined: (i) insulating vinylic polymers@AuNPs; (ii) conducting polymers@AuNPs, and (iii) sol−gel inorganic polymers@AuNPs. V.1.1. Insulating Vinylic Polymers@AuNPs. Vinylic molecularly imprinted polymers were grafted on AuNPs immobilized on a gold electrode by taking advantage of the diazonium salt chemistry,25−31 as depicted in Figure 4.25 The MIPs films synthesized by surface-initiated photopolymerization served for the specific and selective recognition of dopamine with a detection limit down to 0.35 nmol L−1. The high sensitivity of this electrochemical sensor was attributed to the large surface area provided by the embedded AuNPs, along with their catalytic effects. Despite the intrinsic insulating character of vynilic MIPs grafts, it was shown that they can be advantageously used in electrochemical sensors if their thickness is nanometric.32 V.1.2. Conducting Polymers@AuNPs. In order to improve further the conductivity of molecularly imprinted sensors, an interesting approach consists in elaborating MIP films based on conducting polymers doped with metal nanoparticles. For this, poly(aminothiophenol) conducting polymers were combined with gold NPs by various approaches. For example, the stepwise modification of a gold electrode by a thin MIPs film followed by the self-assembly of AuNPs was used to prepare AuNPs/poly-o-

Figure 5. Analysis of TNT by an oligoaniline-cross-linked AuNPs functionalized electrode. Reproduced with permission from ref 36. Copyright 2008 American Chemical Society.

In the latter case, the electrochemical aggregation of AuNPs bridged by oligoaniline units at the Au electrode further increased the sensitivity of the modified electrode as a result of the three-dimensional conductivity of the NPs matrix.36 The limit of detection was found to be of 460 ppt, corresponding to 2 nM of TNT. V.1.3. Sol−Gel Inorganic Polymers@AuNPs. Inorganic SiO2based MIP films prepared from a sol−gel process were proposed to form a confined and rigid structure able to offer superior binding specificity toward the target molecule.37 The advantages of such inorganic polymeric films over organic ones include easier preparation, higher selectivity to the template, and lower nonspecific binding. However, slow diffusion of the analytes across the inorganic MIPs film and low conductivity constitute the major limitations of this approach. In order to improve charge conductivity and mass transportation efficiency, inorganic

Figure 4. Scheme of the procedure to prepare AuNPs@MIPs films on Au electrodes (E-np-MIP): (a) Functionalization of the gold electrode by mercaptobenzene diazonium salt;25 (b) grafting of AuNPs on the electrode via the sulfhydryl moiety of the aryl layer; (c) electrografting of photoinitiator from 4-benzoyl benzene diazonium salt; (d) polymerization and synthesis of AuNPs@MIPs; (e) rebinding and extraction experiments. MAA: methacrylic acid, EGDMA: ethylene glycol dimethacrylate, T: template molecule. Reprinted with permission from ref 25. Copyright 2011 Elsevier. 5468


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Figure 6. Scheme of the procedure to elaborate ITO@AuNPs@MIPs sensor: (a) the hydrated process, (b) functionalization of the surface by mercapto groups using a silanization reaction, (c) self-assembly of AuNPs, (d) preparation of the MIPs film by a sol−gel technique, (e) extraction of imipramine, and (f) rebinding process. Reprinted with permission from ref 38. Copyright 2009 Elsevier.

MIP films were hybridized with gold nanoparticles for detection of imipramine and the azo-dye Sudan I.38,39 The nanocomposite films obtained by this combination were shown to provide a higher surface area, quicker response compared to AuNPs-free films, and an overall enhancement of the electrochemical detection performance. For example, a sensitive and selective electrochemical sensor was fabricated for detection of the antidepressant imipramine via stepwise modification of ITO electrodes by AuNPs and thin MIPs films, as schematized in Figure 6. The excellent performance of the ITO@AuNPs@MIPs electrodes toward imipramine was ascribed to the electrochemical catalytic activities of the AuNPs functional monolayer and the plentiful selective binding sites provided by the porous sol−gel MIPs film. The sensor was shown to respond quickly to imipramine, even after only 1 min of incubation. V.2. AuNPs@MIPs Nanocomposite Thin Films for Gravimetric Sensors. Quartz crystal microbalances (QCMs) detect mass variations at the nanogram level, by measuring the change in frequency of a quartz crystal resonator. QCM devices combined to molecular imprinted polymers are very attractive sensing platforms as they are able to detect very accurately the incorporation of low quantities of analytes. To design MIPsbased QCM sensors, the MIPs are either polymerized in situ on the sensor surface by surface grafting, sandwich casting, spin coating, and electropolymerization or immobilized on the surface using ex situ prepared MIP beads.40 Among these approaches, electropolymerization was used for the construction of MIPsbased QCM sensors due to fast responses and the possibility of miniaturization. Nevertheless, electropolymerized films are dense and compact and exhibit a small proportion of imprinted sites, decreasing their efficiency. In order to improve the specific surface area and the amount of imprinted sites in the MIPs, the group of Kong41 integrated AuNPs into the electropolymerization process. For this, molecularly imprinted poly(o-aminothiophenol) (PoAT) membranes were electrodeposited on a Au electrode surface modified by self-assembled AuNPs. This molecularly imprinted QCM sensor showed good frequency response to the binding of the analytes with improved properties thanks to the introduction of AuNPs. Particularly, it was used for the detection of Ractopamine (RAC), a common β-adrenergic

agonist widely used in animal feed as a growth promoting agent, and it was applied to determine RAC residues in spiked wine samples. The shifts in frequency were proportional to concentrations of RAC in the range of 2.5 × 10−6 to 1.5 × 10−4 mol L−1 with a limit of detection of 1.17 × 10−6 mol L−1. Therefore, such a sensor was shown to be promising thanks to the high specific surface area of AuNPs, the high selectivity of MIPs films, and the high sensitivity of quartz crystal microgravimetry. V.3. AuNPs@MIPs Nanocomposite Thin Films for Optical Sensors. Surface plasmon resonance (SPR) spectroscopy allows the detection of modifications in refractive index occurring at the surface of gold, induced by the recognition of an analyte. The use of this technique for SPR sensors was limited by the small variations in refractive index generated by the binding event. Indeed, only large molecules such as macromolecules or proteins can lead to measurable refractive index variations. In contrast, low-molecular-weight analytes lack the necessary sensitivity, particularly at low coverage. In order to amplify the SPR changes, the surface plasmon wave associated with thin gold films has been coupled with the localized plasmon of AuNPs. This coupling induces a shift of the SPR energy and an enhancement of the SPR changes. In a first simple example of composite films of AuNPs and MIPs for optical sensing, Sugimoto et al. prepared a polymer gel with immobilized AuNPs for the selective colorimetric detection of adrenaline.42 The selective binding of the molecule induced a swelling of the gel network causing a greater distance between the immobilized AuNPs. The increase of the distance between the NPs could be spectroscopically read out by a blue-shift of the absorption spectrum in response to the target molecule. These interesting results encouraged the same authors to apply AuNPs@MIPs composite films to SPR sensors as a recognition and signaling element as depicted in Figure 7.43 Here, the swelling of the imprinted polymer gel triggered by the analyte (dopamine) binding event caused greater distance between the gold nanoparticles and the substrate, shifting the dip of the SPR curve to a higher SPR angle. The AuNPs were shown to be effective for enhancing the signal intensity (the change of SPR angle) by comparison with a sensor chip without gold 5469


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isotropic spherical gold nanoparticles. Biosensing surfaces were prepared from glass substrates coated successively by AuNRs and MIPs layers obtained from siloxane copolymerization (see Figure 8). The resulting sensing surface was then evaluated for the detection of neutrophil gelatinase-associated lipocalin, a biomarker for acute kidney injury. This approach enabled monitoring the imprinting process step-by-step and the detection of the protein at physiologically relevant concentrations. In order to perform the sensing experiments by transmission surface plasmon resonance spectroscopy (T-SPR), Fendler et al.45 used transparent glass substrates covered by gold nanoislands rather than opaque gold planar substrates to deposit the AuNPs@MIPs composite films, as schematized in Figure 9. This simple experimental setup was employed to monitor the changes of the surface plasmon resonance. Such gold nanoparticleenhanced T-SPR spectroscopy was shown to be highly sensitive, versatile, and cost-effective for the detection of cholesterol. Another way to elaborate the AuNPs@MIPs nanocomposite films consisted of electropolymerizing thioaniline-modified AuNPs onto a thionaniline-functionalized Au surface, in the presence of a target molecule46 (see Figure 10). This created bisaniline-cross-linked AuNPs composites that, after the removal of the target molecule, provided molecularly imprinted polymers with high sensitivities. This approach which was previously applied in the case of electrochemical sensors (see paragraph on “Conducting Polymers@AuNPs”) by the same authors, provided ultrasensitive SPR platforms for sensing

Figure 7. Schematic illustration of AuNPs@MIPs-coated SPR sensor chip for the detection of dopamine. Reproduced with permission from ref 42. Copyright 2005 American Chemical Society.

nanoparticles. Furthermore, the analyte binding process and the consequent swelling appeared to be reversible, hence the recovery of the sensor chip. In another example, Singamaneni et al.44 used anisotropic gold nanorods (AuNRs) in order to take advantage of their remarkable properties, which have been shown to exhibit higher refractive index sensitivity compared to

Figure 8. (a) TEM image of AuNRs. (b) Cross-sectional view of the electric field distribution around AuNRs at the extinction maximum of the longitudinal band. The image is obtained by finite-difference time-domain (FDTD) modeling. (c) Illustration of the gold nanorods coated at their extremities by the siloxane copolymer. Panels (b) and (c) show the spatial matching of the imprinted area with the localization of the plasmonic hotspots. (d) AFM image (scan size 400 nm × 400 nm) of the AuNPs@MIPs and (e) corresponding height profile (following the violet line).44 5470


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Figure 9. (A) Scheme of the gold nanoparticle-enhanced T-SPR MIP sensor with gold nanoislands, PGMA binder, MIPs, gold nanoparticles, and the analyte (cholesterol). (B) Corresponding transmission surface plasmon resonance T-SPR spectra in the absence (left-hand side) and presence (righthand side) of cholesterol. Reproduced with permission from ref 45. Copyright 2006 Royal Society of Chemistry.

Figure 10. Imprinting of molecular recognition sites for antibiotic substrates (for example, neomycin) through the electropolymerization of a bisanilinecross-linked AuNPs composite on a Au Surface. Reproduced with permission from ref 46. Copyright 2010 American Chemical Society.

Figure 11. Immobilization procedure of MIP-NPs and BPA-AuNPs on the sensor chip for the binding of free BPA. Reproduced with permission from ref 47. Copyright 2012 American Chemical Society.

antibiotics46 and amino acids21 through chiroselective imprinted sites. All the above-mentioned SPR sensors based on a combination of AuNPs and MIPs possess prism-based configurations,

measuring the angular reflection spectrum for monochromatic light. An alternative approach was developed by Takeuchi et al.47 who proposed to use optical waveguide surface plasmon resonance (OWG-SPR) sensors. In this setup, resonant 5471


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Chemistry of Materials Table 4. General Characteristics of the AuNPs@MIP Nanocomposite Thin Films for Sensors transducer electrochemical

AuNPs@MIPs receptor 2D-assembly of AuNPs coated by insulating vinylic MIPs 2D or self-assembly of AuNPs in insulating sol−gel MIPs 3D-dispersion by electropolymerization of AuNPs in conducting polymer MIPs

Optical

Gravimetric

Composite gel of MIPs/AuNPs 3D-dispersion by electropolymerization of AuNPs in conducting polymer MIPs MIPs gels with embedded AuNPs 2D assembly of AuNPs on quartz crystal and coating by electrodeposited MIPs

methods

analyte

LOD [mol L‑1]

LOD [mol L‑1] in AuNPs-free MIPs

square wave voltammetry

dopamine

0.35 × 10−9

10−8−10−9

differential pulse voltammetry electrochemical impedance spectroscopy; cyclic voltammetry linear sweep voltammetry amperometry; cyclic voltammetry colorimetric detection surface plasmon resonance

imipramine tolazoline

1 × 10−9 10−7

1.12 × 10−6

TNT dopamine adrenaline antibiotics amino acids dopamine ractopamine

2 × 10−10 7.8 × 10−9 5 × 10−6 0.2 × 10−12 1.2 × 10−8 10−9 1.17 × 10−6

quartz crystal microgravimetry

1.5 × 10−9 9.66 × 10−7 5.8 × 10−8 6.23 × 10−7

ref 17, 25, 48 38 33, 49 36, 50 35, 51 22 46, 52 21, 53 42 41

Figure 12. Scheme of the synthesis of molecularly imprinted poly(NIPAM-AAm-VPBA)@Ag hybrid microgels for the detection of glucose by glucoseresponsive−gel-actuated tunable plasmon coupling. Reprinted with permission from ref 24. Copyright 2012 Elsevier.

dispersed in solution. The interest of the colloidal form compared to the thin film format lies in (i) an increase in specific surface area, the particle being in full contact with the solution containing the target molecule, and (ii) their possible use in biomedical conditions for in vivo/in vitro sensing or drug delivery. The different AuNPs@MIP colloidal nanocomposites were considered separately, according to their nature, i.e., microgel or individual core−shell particles. VI.1. Microgels. An original system based on imprinted hybrid microgels in solution was elaborated by the group of Zhou.24 The microgels were made of Ag nanoparticles immobilized in situ in molecularly imprinted poly(NIPAMAAm-VPBA), a glucose-responsive polymeric microgel containing phenylboronic acids. The AgNPs were closed to each other, favoring plasmon coupling (see Figure 12). A variation of glucose concentration from 0 to 20.0 mM was shown to result in a modification of the distance between AgNPs leading to a visually evident color change from yellow to red, allowing the detection of glucose in artificial tear fluids. These hybrid microgels could thus act as “glucose-indicators”, to

reflection spectra were observed due to the interaction of the surface plasmon with the evanescent wave generated by totally reflected light running within the OWG, and these spectra undergo changes when molecules are adsorbed within the near field on the metal thin film surface. A slab-type optical waveguide-based microfluidic SPR measurement system was developed for the detection of bisphenol A (BPA). Label-free detection of small molecules such as BPA could thus be achieved by using the proposed CWG-SPR sensor in conjunction with MIP nanoparticles and BPA-coated AuNPs (see Figure 11). V.4. Summary of the Different AuNPs@MIPs Nanocomposite Thin Films for Sensors. Table 4 presents a general overview of the sensors based on AuNPs@MIPs nanocomposite thin films, contrasting the sensing methods, the target analytes, and the limits of detection (LOD) reached by the sensors, in comparison with classical AuNPs-free MIPs.

VI. COLLOIDAL AuNPs@MIPs NANOCOMPOSITES If a large amount of work has been devoted to the preparation of thin films of AuNPs@MIPs, much fewer studies have focused on the use of such nanocomposites in the form of colloidal particles 5472


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particles with the target molecules inducing a fluorescence quenching. The change in fluorescence was explained by the high complexation geometric shape affinity between DPA molecules and DPA cavities on the Au ligand exchange nanoshells. On the basis of a similar protocol, Au−Ag nanoclusters were also covered with imprinted polymers to determine spore concentrations of Bacillus cereus.55 In other types of core−shell AuNPs@ MIPs particles, the surface imprinting was performed via a sol− gel process. For example, Long et al.56 synthesized a triethoxysilane monomer−template complex with BPA to prepared a MIPs layer anchored on the surface of the AuNPs (see Figure 14). The imprinted molecules could then be removed by a simple thermal reaction to generate the sensor. Its application to the detection by SERS of BPA in real samples (local river in China or a functional drink beverage) demonstrated the potential utility of this approach for rapid and selective detection of estrogen molecules in practical applications. On the basis of a similar type of system with core−shell AuNPs@SiO 2 imprinted composites, the group of Shi constructed an electrochemical sensor for the recognition of dopamine.57 To this end, AuNPs@SiO2-MIPs solutions were dropped on a graphite carbon electrode (GCE), dried, and then analyzed by cyclic voltammetry. The prepared sensor was shown to exhibit a high selectivity toward dopamine in comparison to other interferents and a wide linear range over dopamine concentration with a detection limit down to 2.0 × 10−8 M. Its application to the detection of dopamine in human urine samples demonstrated the interest of this approach for the analysis of complex matrices. VI.2.2. Anisotropic Gold Nanocages. Anisotropic gold nanocages (AuNCs), a novel class of hollow plasmonic nanostructures, were proposed for label-free plasmonic biosensing owing to their highly tunable localized surface plasmon resonance (LSPR) into the near-infrared. This spectral region is particularly interesting as the endogeneous absorption coefficient of living tissue in this region is nearly 2 orders of magnitude smaller than that in the visible range. AuNCs also present large scattering and absorption cross sections which is of potential interest for contrast agent applications, photoacoustic imaging, and photothermal therapy. The group of Singamaneni thus functionalized AuNCs with built-in artificial antibodies produced by the molecular imprinting approach using reversible

determine the glucose levels without the aid of any instrumentation. VI.2. Core−Shell Particles. VI.2.1. Spherical Isotropic AuNPs. Individual core−shell AuNPs@MIPs were prepared by the thiol-ligand capping method with polymerizable methacryloylamidocysteine attached to gold nanoparticles (see Figure 13).19,23,54

Figure 13. Scheme of nanoshell based on DPA template reconstruction on AuNPs; brackets symbolizes cross-linking with EDMA. Reprinted with permission from ref 19. Copyright 2009 Elsevier.

In this method, methacryloyliminodiacetic acid−metal was used as a metal-chelating monomer via metal-coordination− chelation interactions and cholic acid or dipicolinic acid (DPA) as a template. This last molecule is the main participant of Bacillus cereus spores. The sensing was monitored by fluorescence measurements, the interaction of the core−shell

Figure 14. Schematic illustration for fabricating core−shell AuNPs@MIPs particles for selective detection of BPA using a small portable Raman spectrometer. MIP-ir-AuNPs corresponds to the samples from which the template BPA has been removed. Reprinted with permission from ref 56. Copyright 2013 Elsevier. 5473


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Figure 15. (A) Scheme of the procedure for designing AuNCs functionalized by MIPs. (B) Extinction spectra of AuNCs recorded after the different steps of the molecular imprinting process. (C) Corresponding LSPR wavelength shifts, including two cycles of protein release and capture. Reproduced with permission from ref 58. Copyright 2014 RSC.

Figure 16. Schematic illustration of molecular imprinting at the surface of AuNPs/CNT/GC electrode: (1) electrodeposition of AuNPs on the surface of CNT/GC electrode; (2) electropolymerization of o-hydroxyphenol on the surface of AuNPs/CNT/GC electrode; (3) removal/rebinding of TAP on the imprinted sites of the imprinted PHP/AuNPs/CNT/GC electrode. Reprinted with permission from ref 60. Copyright 2012 Elsevier.

surface area, increasing number of imprinted cavities, better electron transfer ability from the binding sites to the surface of the electrode, and excellent catalytic oxidation for the target molecules. For example, the group of Huang prepared electrochemical sensors step by step by first depositing on the electrode some CNT-AuNPs composites for the enhancement of electronic transmission and sensitivity and then the MIP layer. Depending on the nature of the MIP layer, different types of sensors were elaborated. In one example, the sensing layer was composed of β-cyclodextrin-coated CNTs, AuNPs−polyamide amine dendrimers, and chitosan derivatives for the selective and convenient determination of chlortetracycline, a type of broadspectrum antimicrobial drug used in the treatment of human and animal infections.59 Another type of electrochemical sensor was prepared for the detection of pesticide triazophos by deposition of gold nanoparticles on CNTs modified glassy carbon electrode surface

template immobilization and siloxane copolymerization, as shown in Figure 15.58 The colloidal core−shell particles obtained by this approach were evaluated for the detection of kidney injury biomarker down to a concentration of 25 ng·mL−1. The limit of detection achieved by these nanocages was found to be an order of magnitude lower compared to that obtained with AuNRs deposited on a glass substrate.

VII. MULTIHYBRID NANOCOMPOSITES In order to further enhance the physical properties of AuNPs@ MIPs, some authors proposed to incorporate inside the nanocomposites some other types of nanoparticles, in addition to AuNPs, such as carbon nanotubes, graphene, or titanium oxide. VII.1. AuNPs@MIPs with CNTs. The combination of CNTs and AuNPs inside the MIPs matrix allows providing larger 5474


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Figure 17. Scheme of the elaboration (1, 2, and 3) and sensing mechanism (4 and 5) of the photoelectrochemical sensor. Reproduced with permission from ref 63. Copyright 2013 Royal Society of Chemistry.

Table 5. Properties of the Nanomaterials in the Multihybrid Sensors and Resulting Synergy type of multifunctional nanocomposite AuNPs@MIPs combined to CNT

properties of the added nanomaterial

properties of AuNPs large surface-to-volume ratio; good conductivity; catalytic activity; strong adsorption ability; biocompatility

AuNPs@MIPs combined to graphene

AuNPs@MIPs combined to TiO2 nanotubes

good mechanical strength excellent electrical conductivity high chemical stability high surface area photocatalytic activity

synergy/benefits enhancement of electron transfer improvement of the current response and detection sensitivity increase of the electrocatalytic ability

ref 59, 60

61, 62

increase in the effective surface area leading to larger number of imprinted sites. enhancement of the interfacial charge transfer 41, 63 better separation of the photogenerated charges decrease of the destructive effect of uv light

surface of graphene−AuNPs/chitosan−PtNPs/gold electrode through Au−S bonds and hydrogen interactions before grafting the MIPs by electropolymerization of HAuCl4, MNA, and erythromycin. Sensing experiments of erythromycin in real spiked samples were performed evidencing a high selectivity and good reproducibility. VII.3. AuNPs@MIPs with TiO2. The combination of titanium oxide nanoparticles and AuNPs@MIPs composites was shown to offer new opportunities for specific and sensitive detection of target analytes. A simple example is the incorporation of TiO2 nanoparticles to enhance the electron conduction and sensitivity of AuNPs-MIPs nanocomposites for the detection of 4-nonylphenol, an environmentally toxic and potential endocrine disrupting chemical.54 Another interesting illustration is the functionalization of TiO2 nanotubes (NTs) by gold nanoparticles followed by the electropolymerization of ophenylenediamine (o-PD) monomer and chlorpyrifos (a commonly used pesticide), as a template molecule,63 as shown in Figure 17. The fast photoelectronic communication between chlorpyrifos, PoPD, AuNPs, and TiO2 NTs allowed the sensitive sensing of chlorpyrifos in real samples. The mechanism is based on a photoelectric transition in poly(o-phenylenediamine) (PoPD), from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO), releasing excited electrons to the AuNPs, and then to the conduction band of the TiO2 NTs.

using a potentiometric method, followed by electropolymerization of o-hydroxyphenol at the AuNPs/CNT/GC electrode in the presence of the target molecule (see Figure 16). The results indicated that the combination of AuNPs and CNTs could strikingly amplify the electrochemical response of triazophos (TAP) and improve the sensitivity of the sensor.60 VII.2. AuNPs@MIPs with Graphene. The incorporation of graphene with plasmonic nanoparticles was proposed in order to obtain a synergetic effect for improving further the electrochemical response and the effective surface area of the electrode. To this end, the group of Huang prepared a novel imprinted electrochemical sensor based on chitosan−silver nanoparticles/ graphene−carbon nanotubes composites deposited on a gold electrode.61 The molecularly imprinted polymer layer was then electropolymerized in the presence of neomycin (an aminoglycoside antibiotic against Gram-negative bacteria) as the template and pyrrole as the monomer. The performance of the sensor was investigated using cyclic voltammetry and amperometry. Interestingly, the sensor could be applied to determine the neomycin in milk and honey samples with good reproducibility. The same group proposed another type of MIP electrochemical sensor based on multihybrid composites with chitosan−platinum (Pt) nanoparticles combined with graphene−gold nanoparticles.62 For this, erythromycin (an antibiotic) as the template molecule and 2-mercaptonicotinic acid (MNA) as the functional monomer were first assembled on the 5475


Chemistry of Materials Simultaneously, the positively charged hole (h+) of PoPD that took part in the oxidation process was consumed to promote the amplification of the photocurrent response. The measured photocurrents were found to be proportional to the concentrations of chlorpyrifos from 0.05 to 10 mmol L−1 with a detection limit of 0.96 nmol L−1. In addition, the association of all these components in the MIPs matrix was shown to reduce the destructive effect of UV light and the photoholes generated by illuminated TiO2 to template molecules. VII.4. Synergetic Properties of the Multihybrid Nanocomposites. The combination of AuNPs@MIPs with other kinds of nanoparticles was proved to enhance the sensing capacities of the multihybrid-based sensors. Table 5 reports the particular properties of each individual component in the nanocomposites and emphasizes the resulting synergy.

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ACKNOWLEDGMENTS

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REFERENCES

We thank the LabEx SEAM (Science and Engineering for Advanced Materials and devices) of Sorbonne Paris Cité for financial support.

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VIII. CONCLUSION This article provides an up-to-date review on nanocomposites composed of AuNPs and MIPs for (bio)sensing applications. The outstanding properties of AuNPs have shown great potential for the development of analytical hybrid MIP systems with attractive and promising behaviors in term of sensitivity, selectivity, and reliability. The integration of AuNPs into the MIP matrix allows combining and enhancing the properties of inorganic nanoparticles and polymer, thus offering highperformance novel materials with advanced new functions that find applications in many industrial fields. From a physicochemical point of view, the introduction of AuNPs into the MIP matrix has greatly improved the specific surface area of the nanocomposite, which results in easy removal of the analyte and higher accessibility to the recognition sites, larger binding ability, and faster binding kinetics. The electron transfers are also facilitated with the presence of AuNPs while new optical properties are provided by surface plasmon resonance. However, this rapidly growing area is still in its infancy from the viewpoint of practical, real world applications. There remain issues to be addressed before the full potential of such AuNPs@ MIPs nanocomposites for sensing applications can be achieved. Particularly, the design of AuNPs@MIPs with long-term stability in various environments is a significant challenge. Moreover, as the size and shape of AuNPs strongly affect their optical and electrical properties, another promising research direction is the production of AuNPs@MIPs with embedded anisotropic AuNPs (nanorods, bipyrimids, nanocages, etc.). Combining AuNPs with various kinds of nanoparticles is also a compelling direction. Indeed, multicomposites of AuNPs@MIPs with other nanomaterials such as carbon-based (CNT, graphen), oxide (TiO2, Fe2O3, etc.), or metallic (Pt, Pd, etc.) nanoparticles clearly offers distinct added-value properties. In addition to research of new structures or new compositions, attempts to further improve the analytical performance of AuNPs@MIPs at the current juncture, through the prevention of nonspecific adsorption of (bio)molecules or the shortening of analysis time, warrant our full interest. From what has been stated above, definitely the MIP science and technology has entered a new era where the molecular recognition ability of the imprinted polymers and the implementation of nanostructures permit designing unique biomimetic sensor devices with unprecedented analytical performances.

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AUTHOR INFORMATION

Notes

The authors declare no competing financial interest. 5476


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