Fabrication of Biosensing Surfaces Using Adhesive Polydopamine Hunghao Chu Dept. of Anesthesiology, Children’s Hospital Boston, Boston, MA 02115, USA Koch Inst. for Integrative Cancer Research, Massachusetts Inst. of Technology, Cambridge, MA 02139
Chun-Wan Yen Inst. for Medical Engineering and Science, Massachusetts Inst. of Technology, Cambridge, MA 02139
Steven C. Hayden Los Alamos National Laboratory, Materials Physics and Applications Division, Los Alamos, NM 87545 DOI 10.1002/btpr.1991 Published online 00 Month 2014 in Wiley Online Library (wileyonlinelibrary.com)
Dopamine can be induced to polymerize on a variety of substrates, providing a robust and bioinspired surface coating that can be used to tune substrate surface properties and to sequester other species at the interface. We first exploit the facile nature of this surface modification procedure to generate an array of polydopamine that, in conjunction with fluorescent tags, provides the ability to detect multiple protein targets simultaneously and with great specificity. We then demonstrate the use of polydopamine as a matrix to confine gold nanoparticles at the surface of glass and graphene substrates. The nanoparticles (NPs) are used to template further gold nanoparticle growth in situ at the interface; subsequent calcination to remove the polydopamine matrix and sinter the NPs generates a highly active surface enhanced Raman scattering surface that allows for sensitive molecular detection. These varied uses in surface modification/biosensing demonstrate the utility of polydopamine as a functional surface modification for control of physical and electronic properties at the interC 2014 American Institute of Chemical Engineers Biotechnol. Prog., 000:000–000, face. V 2014 Keywords: biosensing, functional surface coating, SERS, polydopamine, gold nanoparticles
Introduction Dopamine is best known as a neurotransmitter associated with many neuronal disorders including Parkinson’s and Alzheimer’s diseases. However, distinct from its importance in medicine, polymerized dopamine (polydopamine) has attracted recent attention due to its intriguing properties as a surface coating.1 Spontaneous polymerization of dopamine under alkaline conditions has been shown to generate a stable surface coating on a variety of substrates ranging from metals to plastics to glass. Although the underlying mechanism is not fully understood, it is generally accepted that oxidation of catechol to quinone under alkaline conditions is followed by subsequent rearrangement and polymerization.2 Polydopamine surface coatings have many advantages in terms of applications, including (1) facile formation through simple immersion of the substrate in alkaline dopamine solution, (2) durability, (3) adjustable thickness via control of reaction time and pH/concentration of the solution,3 (4) control over surface hydrophobicity,4 and roughness,5 and (5) provision of a platform for further surface modifications, such as covalent attachment of amine-containing molecules
Correspondence concerning this article should be addressed to Hunghao Chu at [email protected] C 2014 American Institute of Chemical Engineers V
(e.g., proteins, peptides)6,7 or thiols8,9 via Michael addition to the quinone groups on polydopamine. Polydopamine has also been utilized to decorate nanomaterials by forming nanosized layers on their surfaces, imparting new properties to the materials and extending the range of their applications. For example, a polydopamine layer about 40 nm in thickness has been formed on electrospun polycaprolactone nanofibers, and a 20 nm thick polydopamine layer has been generated on polystyrene nanofibers.10,11 Polydopamine-coated carbon nanotubes and gold nanoparticles (AuNPs) have been shown to be more biocompatible than their uncoated counterparts.12,13 Elsewhere, polydopamine has been used to adsorb then desorb hemoglobin to Fe3O4 nanoparticles (NPs); the imprint left behind in the polydopamine layer facilitates subsequent molecular recognition of hemoglobin.14 Polydopamine has also shown utility in attachment of an anti-epidermal growth factor receptor antibody to gold nanorods, allowing for specific targeting and ablation of cancer cells via the photothermal effect.15 Polydopamine has been employed elsewhere in the construction of metallic nanostructures, where the underlying mechanism involves polydopamine-induced nucleation and reduction of metal ions in situ. The primary examples of NPs produced in this fashion involve the use of gold or silver, with final NP size being governed by the concentration of metal salt (silver nitrate, chloroauric acid, respectively).16,17 For 1
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example, spherical silver nanoparticles (AgNPs) with diameters of 30–50 nm have been generated on polydopaminecoated cellulose nanofibers,18 and AgNPs of about 100 nm diameter have been generated on polydopamine-coated silica particles.19 Similarly, spherical and triangular AuNPs with sizes between 20 and 30 nm have been grown on polydopamine-coated carbon nanotubes,20 and the growth of 40 nm diameter AuNPs has been induced within the channels of a poly(dimethylsiloxane) microchip.21 In this study, we examined the use of polydopamine in surface modification with two applications in biosensing, including: (1) a multiplexed fluorescence-based detection platform and (2) a AuNPs-layer for surface enhanced Raman scattering (SERS) detection. As a fluorescence-based detection platform, protein detection was achieved using an array of polydopamine dots deposited on a hydrophobic substrate. The polydopamine dot formed a hydrophilic barrier with the hydrophobic region, which greatly reduced the consumption of reagents. The polydopamine matrix in conjunction with fluorescent antibody tags allowed for the needed specificity to detect model protein targets in the presence of other nontarget proteins. For use in a SERS detection platform, polydopamine was used to template nanostructured gold films on a variety of surfaces in the construction of SERS substrates. Calcination allowed for removal of the polydopamine matrix and yielded a substrate on which the demonstrable enhancement of Raman bands was sufficient to detect a protein with a normally weak Raman signal.
Materials and Methods Materials Dopamine hydrochloride, ascorbic acid, bovine serum albumin (BSA), chloroauric acid (Sigma Aldrich, St. Louis, MO), mouse IgG, rabbit IgG (Santa Cruz Biotechnology, R 488 conjugated goat anti-mouse Dallas, TX), Alexa FluorV R 568 conjugated donkey anti-rabbit IgG IgG, Alexa FluorV (Life Technologies, Carlsbad, CA), and rhodamine 6G (Exciton, Dayton, OH) were used directly without purification. Four-layer graphene sheets were prepared on coverslips by the mechanical exfoliation method.22 Formation of polydopamine dots To form polydopamine dots, 1 lL of dopamine solution (0.5 mg/mL) prepared in Tris buffer (pH 8.5) was dropped onto the substrate and allowed to dry overnight at ambient temperature. The dry sample was washed three times with distilled water, dried again with air, and stored at ambient temperature until use. Fluorescence-based protein detection on polydopamine dots Four protein solutions were prepared in phosphate buffer saline (PBS): (1) BSA solution (100 lg/lL), (2) mouse IgG diluted in BSA solution (mass ratio 1:122.5), (3) rabbit IgG diluted in BSA solution (1:122.5), and (iv) mouse IgG and rabbit IgG diluted in BSA solution (1:1:122.5). An aliquot (1 lL) of each solution was deposited on each polydopamine dot. After overnight incubation at 4 C, the coating was washed three times with PBS. Substrates were subsequently R 488 conjusubmerged in fluorophore solutions (Alexa FluorV R 568 conjugated gated goat anti-mouse IgG and Alexa FluorV donkey anti-rabbit IgG) for 1 h at ambient temperature and
washed three times with PBS. Bright-field and fluorescent images were captured by a CTR 6000 microscope (Leica Microsystems, Wetzlar, Germany) equipped with ImagePro 6.2 Software (Media Cybernetics). AuNP deposition on polydopamine dots Colloidal AuNPs capped by citrate molecules were prepared by the established method, and imaged by a CM-200T transmission electron microscope (TEM) (FEI, Hillsboro, Oregon).23 A solution containing AuNPs (2 nM) and dopamine (0.5 mg/mL) was deposited on the glass coverslip and incubated overnight at room temperature. After washing, equal volumes of chloroauric acid (3 mM) and ascorbic acid (10 mM) were added sequentially to form AuNPs. The same steps were repeated five times. Calcination was performed by heating on a digital hotplate (VWR International, Radnor, PA) for 2 h at 400 or 500 C. Scanning electron microscopy All samples for bright field and scanning electron microscopy (SEM) were prepared as described above then air-dried and sputtered with gold to form an 8-nm thick layer. Samples were imaged using a QuantaTM scanning electron microscope (10 kV, 3 nm spot size) (FEI, Hillsboro, Oregon). Surface enhanced Raman spectroscopy Samples for SERS were preparing by spin-coating R6G (0.1 M) or BSA (10 lg/lL) on the coverslip. Spectra were acquired using an inVia Raman microscope (Renishaw) equipped with an argon laser (514 nm).
Results and Discussion Polydopamine provides a simple means of converting a hydrophobic surface into a hydrophilic surface; therefore appropriate deposition methods could allow for its use in constructing a platform with both hydrophilic and hydrophobic character. A detection platform composed of hydrophilic and hydrophobic regions has myriad advantages such as a high signal-to-noise ratio; fabrication of such arrays has therefore become a crucial objective in the area of biosensing.24 To illustrate several components of such an approach, we constructed a surface for fluorescence detection by forming polydopamine dots in the wells of a polystyrene 96-well plate to generate protein-adhesive surfaces. Polydopamine dots were formed by dropwise deposition and subsequent polymerization of monomeric dopamine on the substrate. In lieu of coating the entire surface with polydopamine via submersion in dopamine solution, this dropwise deposition strategy uses less material, allows for control over the spatial distribution of polydopamine, and generates a surface with discrete hydrophilic and hydrophobic areas. Following exposure to IgG protein, a fluorophore-conjugated antibody indicated that only those parts of the polystyrene surface that were coated with polydopamine efficiently absorbed protein (Figure 1A). These results implicate the utility of polydopamine in the modification of surface hydrophobicity. To investigate further, we formed polydopamine dots on highlyhydrophobic paraffin wax film (contact angle of water on paraffin wax 104 ). The hydrophobicity of the paraffin wax substrate severely limited the ability of the deposited dopamine solution to spread across the surface. This was
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Figure 1. A: Schematic representation of polydopamine (PD) dots in a polystyrene (PS) 96-well plate. B: Photographs of polydopamine dots on a paraffin wax substrate. Top: Polydopamine dots formed by increasing volumes of dopamine solution (1, 2, 3, 4, 5, 6, 7, 8, and 10 lL). Bottom: A 535 matrix array of PD dots formed within an area of 1 square inch. C: Fluorescent images (203) showing: (BSA) lack of nonspecific interactions with a non-target protein or with the PD matrix or substrate, (BSA/M-IgG) successful retention and selective detection of M-IgG, (BSA/R-IgG) successful retention and selective detection of R-IgG, and (BSA/M-IgG/R-IgG) successful detection of two target proteins simultaneously.
advantageous and allowed not only for control over the dot size of deposited dopamine (Figure 1B), but also for the facile formation of arrays of dots in a small area (Figure 1C). Dried polydopamine dot arrays were found to be suitably stable for long-term storage, indicating potential for off-theshelf availability. The polydopamine coating exhibited a high affinity for glass, as negligible polydopamine was removed from the surface even after extensive washing. We next investigated the ability of polydopamine dot arrays to facilitate detection of specific protein targets via protein retention and tagging. To this end, we exposed polydopamine dots to one of four protein solutions: (1) BSA, (2) BSA/Mouse IgG, (3) BSA/Rabbit IgG, or (4) BSA/Mouse IgG/Rabbit IgG. BSA was used both as a control and to avoid non-specific binding. Following incubation, the dots were washed and briefly exposed to a solution of fluorophore-labeled anti-mouse and anti-rabbit IgG (Figure 1C). No signal was observed for the BSA control (Group 1), no anti-rabbit signal was observed for the BSA/mouse group (Group 2), and no anti-mouse signal was observed for the
BSA/rabbit group (Group 3). These observations indicated a lack of nonspecific binding of the antibody tags to either the paraffin wax substrate or to the polydopamine-coated surface. Group 4 (BSA/mouse/rabbit) returned both green and red fluorescent signals, indicating the ability of one dot to detect multiple targets simultaneously. Further, since each polydopamine dot can serve as an independent sensor, this platform allows for multiple sensing arrays to be contained in a single device. We are currently mimicking the procedures of enzyme-linked immunosorbent assay (ELISA) to investigate the use of this platform to detect more targets in a more complex environment. The ultimate goal is to greatly shorten the assay time and reduce consumption of reagents. We next investigated the use of polydopamine in the construction of a surface for SERS detection.25 SERS is one of the few techniques that is extremely sensitive to molecular binding.26 A previous study constructed a SERS detection platform by using a surface coating of polydopamine to reduce Au31 ions, initiating formation of a superficial Au layer.20 We anticipated the process to be more efficient and
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Figure 2. SEM images showing the morphology of the polydopamine and PD-AuNPs coatings on glass coverslips. A, B: SEM images of a polydopamine dot. C: UV–vis spectrum and TEM image insert of the AuNPs. D–F: SEM images of PD-AuNPs on coverslips.
Figure 3. Morphology of AuNP clusters. Bright-field images of (A) PD-AuNPs and (B) AuNP clusters. C, D: SEM images of AuNP clusters.
controllable if AuNPs were immobilized upon polymerization of dopamine and if a reducing agent were used to facilitate Au layer deposition.
Pre-formed AuNPs of 10–20 nm diameter were combined with dopamine solution, and the dopamine solution was polymerized to form a polydopamine dot with embedded
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Figure 4. Morphology and Raman spectra of calcined AuNPs. SEM images of the AuNPs calcined at (A) 400 C and (B) 500 C. C: Raman spectra before and after calcination. (D) Raman spectra of R6G on 400 C and 500 C calcined AuNPs.
AuNPs (PD-AuNPs). Bright-field and SEM images of the polydopamine surface revealed a flat surface with scattered aggregates ranging in size from sub-micron to several microns (Figures 2A,B). TEM and absorption spectroscopy were used to characterize the AuNPs (Figure 2C). The low magnification SEM image of the PD-AuNPs composite revealed flat regions similar to those observed in polydopamine alone as well as more numerous aggregates than observed in the absence of AuNPs (Figure 2D). The high magnification images revealed that the aggregates contained particle-like features mostly sized below 50 nm (Figures 2E,F). The matrix-embedded AuNPs were then used as seeds to template deposition of more AuNPs from gold salt. When gold salt was added alone, no deposition or NP formation was observed. However, when a mild reducing agent, ascorbic acid, was added as well, a color change to purple was observed immediately, indicating the formation of AuNPs. The same procedure was repeated five times until the color became dark purple. The bright-field images indicated that this procedure efficiently covered the majority of the polydopamine dot surface with AuNP clusters (Figures 3A,B). The SEM images showed that the AuNPs were mostly smaller than 100 nm in diameter and were interconnected (Figures 3C,D), indicating that this approach is an efficient method to generate AuNPs on the polydopamine dots. While scaffolding of AuNPs on the surface offers an ideal platform for SERS with potential use in molecular detec-
tion,27,28 polydopamine itself has strong Raman signals in the region of 1200–1700 cm21. Many molecules possess signature signals in this region, so the presence of polydopamine in the SERS platform could substantially compromise the detection capability essential to the detection of many molecules with weak Raman signals. To circumvent this potential drawback, we removed polydopamine by calcination. According to the previous thermogravimetric study,18 pure polydopamine began to decompose when the temperature was above 200 C, and 75% of polydopamine was removed when the temperature reached 535 C. In our study, we examined calcination at 400 C and 500 C. AuNPs were not removed by calcination but rather underwent extensive fusing. The SEM images pointed out a higher degree of sintering at higher temperatures, consistent with previous observations (Figures 4A,B).29 At both calcination temperatures, we observed a complete disappearance of the dopamine Raman signals between 1200 and 1700 cm21 as well as an absence of polydopamine under bright-field microscopy, suggesting that polydopamine was completely removed at both 400 and 500 C (Figure 4C). To ascertain the utility of these surfaces for SERS detection, spin coated R6G on the various substrates and measured the Raman spectra for each. We observed significant signal enhancement for both calcined AuNP samples and no signal for R6G on glass (Figure 4D). Intriguingly, the substrate that was calcined at 500 C also gave a stronger enhancement of R6G Raman bands.
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Figure 5. Au nanostructures on graphene surface. SEM images of (A) PD-AuNPs, (B) AuNP clusters, and (C) calcined AuNPs on the graphene surface. D: Raman spectra of graphene before and after calcination. E: Raman spectra of R6G on different substrates examined by two laser intensities. (Top: 15 mW, Bottom: 1.5 mW). F: Raman spectra illustrating the difference between R6G signals on calcined AuNPs vs. on bare graphene.
Figure 6. A: Raman spectra of the calcined AuNPs substrate on graphene, BSA deposited on glass, and BSA deposited on the AuNP substrate. B: Raman spectra illustrating the difference between bare graphene and BSA on graphene. C: Zoom view of Raman spectrum of BSA on graphene.
We next examined a AuNP-based SERS platform on a graphene substrate, both to demonstrate the versatility of this approach to SERS platform construction as well as to exploit the unique optical and electronic properties of graphene. Graphene has great potential for diverse usage,30,31 and significant effort has been directed toward the incorporation of nanostructures into graphene-based materials in order to develop further the properties of the composite materials and to extend their applications.32 In traditional deposition of AuNPs onto graphene surfaces, the surface is usually functionalized with ligands that have an affinity for gold in order to sequester the NPs at the surface.33,34 Our use of polydopamine allowed for circumvention of the surface modification step, as pre-formed AuNPs in dopamine solution can be polymerized directly onto the graphene surface (Figure 5A).
Subsequent addition of chloroauric acid and ascorbic acid templates produced more AuNPs onto the surface in situ. SEM revealed that the AuNPs deposited onto graphene in this fashion are larger in size than those deposited onto glass in the same manner, although the cause of this difference in size needs further investigation. Following deposition, polydopamine was removed via calcination at 400 C to generate a non-interfering, Raman-active surface. Calcination at 500 C was not considered for this sample, as high temperatures are prone to generate defects in graphene.35 Regardless, calcination at 400 C proved sufficient to remove Raman traces of polydopamine, while the structure of graphene was mostly preserved (Figure 5D). Prior studies have found that, as a substrate, graphene is capable of enhancing Raman signals of the deposited
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analytes.36,37 We directly examined the SERS effect of the calcined AuNPs on graphene as well as the SERS effect of graphene alone (Figure 5E). We found that both AuNPs on graphene as well as bare graphene enhanced the Raman signals of R6G. However, whereas AuNPs deposited on graphene provided enhancement at both high and low laser intensities, the SERS effect in graphene was much less pronounced at the lower laser intensity. We observed some minor variances in the Raman band structure of R6G on the two surfaces, variances that likely result from a difference in electronic properties. Whereas a previous report found that the composite material containing AuNPs and graphene showed a higher performance in biosensing due to the electrochemical properties,38 our result indicated that the similar composite material did not offer extra benefit in SERS detection compared to AuNPs or graphene alone. R6G is widely used in Raman studies due to its high Raman intensity, so we next examined the ability of our substrate to detect a molecule with weak Raman scattering: BSA. When BSA was deposited on glass, only Raman signals from the glass substrate itself were present in the spectrum (Figure 6A). However, when BSA was deposited on calcined AuNPs (Figure 6A) or on graphene alone (Figures 6B,C), the SERS effect rendered its Raman signals quite strong. Calcination is an important step to generate a clean SERS surface; if polydopamine were not removed, detection of molecules with weak Raman signals would be difficult due to the strong signals of the polydopamine itself and the steric restriction of analyte access to the gold surface where the signal enhancement is highest.
Conclusions We have demonstrated the utility of polydopamine as a functional surface coating. Polydopamine was found to provide a facile method of controlling surface hydrophobicity, and in conjunction with fluorescently labeled tags, polydopamine allowed for the simultaneous and selective detection of specific protein targets. We also demonstrated the ability of polydopamine to sequester a templating array of AuNPs on a surface without the requirement of expensive machinery, such as an electron beam. Sequential deposition of subsequent layers of AuNPs via facile, in situ formation on the deposited surface was achieved with the aid of a mild reductant. Following calcination to remove the polydopamine matrix, the resulting AuNP-coated substrates demonstrated strong SERS enhancement not only of a traditional Raman marker but also of a model protein. Deposition of these arrays was achieved on glass as well as on the SERS-active graphene, demonstrating the adaptability of this technique to various substrate types. Overall, polydopamine seems to hold great potential in the fabrication of functional surface coatings with emergent properties.
Acknowledgement The authors thank Dr. Madhavi Kadakia for reagents and the fluorescent microscope.
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Chun-Wan Yen Inst. for Medical Engineering and Science, Massachusetts Inst. of Technology, Cambridge, MA 02139
Steven C. Hayden Los Alamos National Laboratory, Materials Physics and Applications Division, Los Alamos, NM 87545 DOI 10.1002/btpr.1991 Published online 00 Month 2014 in Wiley Online Library (wileyonlinelibrary.com)
Dopamine can be induced to polymerize on a variety of substrates, providing a robust and bioinspired surface coating that can be used to tune substrate surface properties and to sequester other species at the interface. We first exploit the facile nature of this surface modification procedure to generate an array of polydopamine that, in conjunction with fluorescent tags, provides the ability to detect multiple protein targets simultaneously and with great specificity. We then demonstrate the use of polydopamine as a matrix to confine gold nanoparticles at the surface of glass and graphene substrates. The nanoparticles (NPs) are used to template further gold nanoparticle growth in situ at the interface; subsequent calcination to remove the polydopamine matrix and sinter the NPs generates a highly active surface enhanced Raman scattering surface that allows for sensitive molecular detection. These varied uses in surface modification/biosensing demonstrate the utility of polydopamine as a functional surface modification for control of physical and electronic properties at the interC 2014 American Institute of Chemical Engineers Biotechnol. Prog., 000:000–000, face. V 2014 Keywords: biosensing, functional surface coating, SERS, polydopamine, gold nanoparticles
Introduction Dopamine is best known as a neurotransmitter associated with many neuronal disorders including Parkinson’s and Alzheimer’s diseases. However, distinct from its importance in medicine, polymerized dopamine (polydopamine) has attracted recent attention due to its intriguing properties as a surface coating.1 Spontaneous polymerization of dopamine under alkaline conditions has been shown to generate a stable surface coating on a variety of substrates ranging from metals to plastics to glass. Although the underlying mechanism is not fully understood, it is generally accepted that oxidation of catechol to quinone under alkaline conditions is followed by subsequent rearrangement and polymerization.2 Polydopamine surface coatings have many advantages in terms of applications, including (1) facile formation through simple immersion of the substrate in alkaline dopamine solution, (2) durability, (3) adjustable thickness via control of reaction time and pH/concentration of the solution,3 (4) control over surface hydrophobicity,4 and roughness,5 and (5) provision of a platform for further surface modifications, such as covalent attachment of amine-containing molecules
Correspondence concerning this article should be addressed to Hunghao Chu at [email protected] C 2014 American Institute of Chemical Engineers V
(e.g., proteins, peptides)6,7 or thiols8,9 via Michael addition to the quinone groups on polydopamine. Polydopamine has also been utilized to decorate nanomaterials by forming nanosized layers on their surfaces, imparting new properties to the materials and extending the range of their applications. For example, a polydopamine layer about 40 nm in thickness has been formed on electrospun polycaprolactone nanofibers, and a 20 nm thick polydopamine layer has been generated on polystyrene nanofibers.10,11 Polydopamine-coated carbon nanotubes and gold nanoparticles (AuNPs) have been shown to be more biocompatible than their uncoated counterparts.12,13 Elsewhere, polydopamine has been used to adsorb then desorb hemoglobin to Fe3O4 nanoparticles (NPs); the imprint left behind in the polydopamine layer facilitates subsequent molecular recognition of hemoglobin.14 Polydopamine has also shown utility in attachment of an anti-epidermal growth factor receptor antibody to gold nanorods, allowing for specific targeting and ablation of cancer cells via the photothermal effect.15 Polydopamine has been employed elsewhere in the construction of metallic nanostructures, where the underlying mechanism involves polydopamine-induced nucleation and reduction of metal ions in situ. The primary examples of NPs produced in this fashion involve the use of gold or silver, with final NP size being governed by the concentration of metal salt (silver nitrate, chloroauric acid, respectively).16,17 For 1
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example, spherical silver nanoparticles (AgNPs) with diameters of 30–50 nm have been generated on polydopaminecoated cellulose nanofibers,18 and AgNPs of about 100 nm diameter have been generated on polydopamine-coated silica particles.19 Similarly, spherical and triangular AuNPs with sizes between 20 and 30 nm have been grown on polydopamine-coated carbon nanotubes,20 and the growth of 40 nm diameter AuNPs has been induced within the channels of a poly(dimethylsiloxane) microchip.21 In this study, we examined the use of polydopamine in surface modification with two applications in biosensing, including: (1) a multiplexed fluorescence-based detection platform and (2) a AuNPs-layer for surface enhanced Raman scattering (SERS) detection. As a fluorescence-based detection platform, protein detection was achieved using an array of polydopamine dots deposited on a hydrophobic substrate. The polydopamine dot formed a hydrophilic barrier with the hydrophobic region, which greatly reduced the consumption of reagents. The polydopamine matrix in conjunction with fluorescent antibody tags allowed for the needed specificity to detect model protein targets in the presence of other nontarget proteins. For use in a SERS detection platform, polydopamine was used to template nanostructured gold films on a variety of surfaces in the construction of SERS substrates. Calcination allowed for removal of the polydopamine matrix and yielded a substrate on which the demonstrable enhancement of Raman bands was sufficient to detect a protein with a normally weak Raman signal.
Materials and Methods Materials Dopamine hydrochloride, ascorbic acid, bovine serum albumin (BSA), chloroauric acid (Sigma Aldrich, St. Louis, MO), mouse IgG, rabbit IgG (Santa Cruz Biotechnology, R 488 conjugated goat anti-mouse Dallas, TX), Alexa FluorV R 568 conjugated donkey anti-rabbit IgG IgG, Alexa FluorV (Life Technologies, Carlsbad, CA), and rhodamine 6G (Exciton, Dayton, OH) were used directly without purification. Four-layer graphene sheets were prepared on coverslips by the mechanical exfoliation method.22 Formation of polydopamine dots To form polydopamine dots, 1 lL of dopamine solution (0.5 mg/mL) prepared in Tris buffer (pH 8.5) was dropped onto the substrate and allowed to dry overnight at ambient temperature. The dry sample was washed three times with distilled water, dried again with air, and stored at ambient temperature until use. Fluorescence-based protein detection on polydopamine dots Four protein solutions were prepared in phosphate buffer saline (PBS): (1) BSA solution (100 lg/lL), (2) mouse IgG diluted in BSA solution (mass ratio 1:122.5), (3) rabbit IgG diluted in BSA solution (1:122.5), and (iv) mouse IgG and rabbit IgG diluted in BSA solution (1:1:122.5). An aliquot (1 lL) of each solution was deposited on each polydopamine dot. After overnight incubation at 4 C, the coating was washed three times with PBS. Substrates were subsequently R 488 conjusubmerged in fluorophore solutions (Alexa FluorV R 568 conjugated gated goat anti-mouse IgG and Alexa FluorV donkey anti-rabbit IgG) for 1 h at ambient temperature and
washed three times with PBS. Bright-field and fluorescent images were captured by a CTR 6000 microscope (Leica Microsystems, Wetzlar, Germany) equipped with ImagePro 6.2 Software (Media Cybernetics). AuNP deposition on polydopamine dots Colloidal AuNPs capped by citrate molecules were prepared by the established method, and imaged by a CM-200T transmission electron microscope (TEM) (FEI, Hillsboro, Oregon).23 A solution containing AuNPs (2 nM) and dopamine (0.5 mg/mL) was deposited on the glass coverslip and incubated overnight at room temperature. After washing, equal volumes of chloroauric acid (3 mM) and ascorbic acid (10 mM) were added sequentially to form AuNPs. The same steps were repeated five times. Calcination was performed by heating on a digital hotplate (VWR International, Radnor, PA) for 2 h at 400 or 500 C. Scanning electron microscopy All samples for bright field and scanning electron microscopy (SEM) were prepared as described above then air-dried and sputtered with gold to form an 8-nm thick layer. Samples were imaged using a QuantaTM scanning electron microscope (10 kV, 3 nm spot size) (FEI, Hillsboro, Oregon). Surface enhanced Raman spectroscopy Samples for SERS were preparing by spin-coating R6G (0.1 M) or BSA (10 lg/lL) on the coverslip. Spectra were acquired using an inVia Raman microscope (Renishaw) equipped with an argon laser (514 nm).
Results and Discussion Polydopamine provides a simple means of converting a hydrophobic surface into a hydrophilic surface; therefore appropriate deposition methods could allow for its use in constructing a platform with both hydrophilic and hydrophobic character. A detection platform composed of hydrophilic and hydrophobic regions has myriad advantages such as a high signal-to-noise ratio; fabrication of such arrays has therefore become a crucial objective in the area of biosensing.24 To illustrate several components of such an approach, we constructed a surface for fluorescence detection by forming polydopamine dots in the wells of a polystyrene 96-well plate to generate protein-adhesive surfaces. Polydopamine dots were formed by dropwise deposition and subsequent polymerization of monomeric dopamine on the substrate. In lieu of coating the entire surface with polydopamine via submersion in dopamine solution, this dropwise deposition strategy uses less material, allows for control over the spatial distribution of polydopamine, and generates a surface with discrete hydrophilic and hydrophobic areas. Following exposure to IgG protein, a fluorophore-conjugated antibody indicated that only those parts of the polystyrene surface that were coated with polydopamine efficiently absorbed protein (Figure 1A). These results implicate the utility of polydopamine in the modification of surface hydrophobicity. To investigate further, we formed polydopamine dots on highlyhydrophobic paraffin wax film (contact angle of water on paraffin wax 104 ). The hydrophobicity of the paraffin wax substrate severely limited the ability of the deposited dopamine solution to spread across the surface. This was
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Figure 1. A: Schematic representation of polydopamine (PD) dots in a polystyrene (PS) 96-well plate. B: Photographs of polydopamine dots on a paraffin wax substrate. Top: Polydopamine dots formed by increasing volumes of dopamine solution (1, 2, 3, 4, 5, 6, 7, 8, and 10 lL). Bottom: A 535 matrix array of PD dots formed within an area of 1 square inch. C: Fluorescent images (203) showing: (BSA) lack of nonspecific interactions with a non-target protein or with the PD matrix or substrate, (BSA/M-IgG) successful retention and selective detection of M-IgG, (BSA/R-IgG) successful retention and selective detection of R-IgG, and (BSA/M-IgG/R-IgG) successful detection of two target proteins simultaneously.
advantageous and allowed not only for control over the dot size of deposited dopamine (Figure 1B), but also for the facile formation of arrays of dots in a small area (Figure 1C). Dried polydopamine dot arrays were found to be suitably stable for long-term storage, indicating potential for off-theshelf availability. The polydopamine coating exhibited a high affinity for glass, as negligible polydopamine was removed from the surface even after extensive washing. We next investigated the ability of polydopamine dot arrays to facilitate detection of specific protein targets via protein retention and tagging. To this end, we exposed polydopamine dots to one of four protein solutions: (1) BSA, (2) BSA/Mouse IgG, (3) BSA/Rabbit IgG, or (4) BSA/Mouse IgG/Rabbit IgG. BSA was used both as a control and to avoid non-specific binding. Following incubation, the dots were washed and briefly exposed to a solution of fluorophore-labeled anti-mouse and anti-rabbit IgG (Figure 1C). No signal was observed for the BSA control (Group 1), no anti-rabbit signal was observed for the BSA/mouse group (Group 2), and no anti-mouse signal was observed for the
BSA/rabbit group (Group 3). These observations indicated a lack of nonspecific binding of the antibody tags to either the paraffin wax substrate or to the polydopamine-coated surface. Group 4 (BSA/mouse/rabbit) returned both green and red fluorescent signals, indicating the ability of one dot to detect multiple targets simultaneously. Further, since each polydopamine dot can serve as an independent sensor, this platform allows for multiple sensing arrays to be contained in a single device. We are currently mimicking the procedures of enzyme-linked immunosorbent assay (ELISA) to investigate the use of this platform to detect more targets in a more complex environment. The ultimate goal is to greatly shorten the assay time and reduce consumption of reagents. We next investigated the use of polydopamine in the construction of a surface for SERS detection.25 SERS is one of the few techniques that is extremely sensitive to molecular binding.26 A previous study constructed a SERS detection platform by using a surface coating of polydopamine to reduce Au31 ions, initiating formation of a superficial Au layer.20 We anticipated the process to be more efficient and
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Figure 2. SEM images showing the morphology of the polydopamine and PD-AuNPs coatings on glass coverslips. A, B: SEM images of a polydopamine dot. C: UV–vis spectrum and TEM image insert of the AuNPs. D–F: SEM images of PD-AuNPs on coverslips.
Figure 3. Morphology of AuNP clusters. Bright-field images of (A) PD-AuNPs and (B) AuNP clusters. C, D: SEM images of AuNP clusters.
controllable if AuNPs were immobilized upon polymerization of dopamine and if a reducing agent were used to facilitate Au layer deposition.
Pre-formed AuNPs of 10–20 nm diameter were combined with dopamine solution, and the dopamine solution was polymerized to form a polydopamine dot with embedded
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Figure 4. Morphology and Raman spectra of calcined AuNPs. SEM images of the AuNPs calcined at (A) 400 C and (B) 500 C. C: Raman spectra before and after calcination. (D) Raman spectra of R6G on 400 C and 500 C calcined AuNPs.
AuNPs (PD-AuNPs). Bright-field and SEM images of the polydopamine surface revealed a flat surface with scattered aggregates ranging in size from sub-micron to several microns (Figures 2A,B). TEM and absorption spectroscopy were used to characterize the AuNPs (Figure 2C). The low magnification SEM image of the PD-AuNPs composite revealed flat regions similar to those observed in polydopamine alone as well as more numerous aggregates than observed in the absence of AuNPs (Figure 2D). The high magnification images revealed that the aggregates contained particle-like features mostly sized below 50 nm (Figures 2E,F). The matrix-embedded AuNPs were then used as seeds to template deposition of more AuNPs from gold salt. When gold salt was added alone, no deposition or NP formation was observed. However, when a mild reducing agent, ascorbic acid, was added as well, a color change to purple was observed immediately, indicating the formation of AuNPs. The same procedure was repeated five times until the color became dark purple. The bright-field images indicated that this procedure efficiently covered the majority of the polydopamine dot surface with AuNP clusters (Figures 3A,B). The SEM images showed that the AuNPs were mostly smaller than 100 nm in diameter and were interconnected (Figures 3C,D), indicating that this approach is an efficient method to generate AuNPs on the polydopamine dots. While scaffolding of AuNPs on the surface offers an ideal platform for SERS with potential use in molecular detec-
tion,27,28 polydopamine itself has strong Raman signals in the region of 1200–1700 cm21. Many molecules possess signature signals in this region, so the presence of polydopamine in the SERS platform could substantially compromise the detection capability essential to the detection of many molecules with weak Raman signals. To circumvent this potential drawback, we removed polydopamine by calcination. According to the previous thermogravimetric study,18 pure polydopamine began to decompose when the temperature was above 200 C, and 75% of polydopamine was removed when the temperature reached 535 C. In our study, we examined calcination at 400 C and 500 C. AuNPs were not removed by calcination but rather underwent extensive fusing. The SEM images pointed out a higher degree of sintering at higher temperatures, consistent with previous observations (Figures 4A,B).29 At both calcination temperatures, we observed a complete disappearance of the dopamine Raman signals between 1200 and 1700 cm21 as well as an absence of polydopamine under bright-field microscopy, suggesting that polydopamine was completely removed at both 400 and 500 C (Figure 4C). To ascertain the utility of these surfaces for SERS detection, spin coated R6G on the various substrates and measured the Raman spectra for each. We observed significant signal enhancement for both calcined AuNP samples and no signal for R6G on glass (Figure 4D). Intriguingly, the substrate that was calcined at 500 C also gave a stronger enhancement of R6G Raman bands.
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Figure 5. Au nanostructures on graphene surface. SEM images of (A) PD-AuNPs, (B) AuNP clusters, and (C) calcined AuNPs on the graphene surface. D: Raman spectra of graphene before and after calcination. E: Raman spectra of R6G on different substrates examined by two laser intensities. (Top: 15 mW, Bottom: 1.5 mW). F: Raman spectra illustrating the difference between R6G signals on calcined AuNPs vs. on bare graphene.
Figure 6. A: Raman spectra of the calcined AuNPs substrate on graphene, BSA deposited on glass, and BSA deposited on the AuNP substrate. B: Raman spectra illustrating the difference between bare graphene and BSA on graphene. C: Zoom view of Raman spectrum of BSA on graphene.
We next examined a AuNP-based SERS platform on a graphene substrate, both to demonstrate the versatility of this approach to SERS platform construction as well as to exploit the unique optical and electronic properties of graphene. Graphene has great potential for diverse usage,30,31 and significant effort has been directed toward the incorporation of nanostructures into graphene-based materials in order to develop further the properties of the composite materials and to extend their applications.32 In traditional deposition of AuNPs onto graphene surfaces, the surface is usually functionalized with ligands that have an affinity for gold in order to sequester the NPs at the surface.33,34 Our use of polydopamine allowed for circumvention of the surface modification step, as pre-formed AuNPs in dopamine solution can be polymerized directly onto the graphene surface (Figure 5A).
Subsequent addition of chloroauric acid and ascorbic acid templates produced more AuNPs onto the surface in situ. SEM revealed that the AuNPs deposited onto graphene in this fashion are larger in size than those deposited onto glass in the same manner, although the cause of this difference in size needs further investigation. Following deposition, polydopamine was removed via calcination at 400 C to generate a non-interfering, Raman-active surface. Calcination at 500 C was not considered for this sample, as high temperatures are prone to generate defects in graphene.35 Regardless, calcination at 400 C proved sufficient to remove Raman traces of polydopamine, while the structure of graphene was mostly preserved (Figure 5D). Prior studies have found that, as a substrate, graphene is capable of enhancing Raman signals of the deposited
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analytes.36,37 We directly examined the SERS effect of the calcined AuNPs on graphene as well as the SERS effect of graphene alone (Figure 5E). We found that both AuNPs on graphene as well as bare graphene enhanced the Raman signals of R6G. However, whereas AuNPs deposited on graphene provided enhancement at both high and low laser intensities, the SERS effect in graphene was much less pronounced at the lower laser intensity. We observed some minor variances in the Raman band structure of R6G on the two surfaces, variances that likely result from a difference in electronic properties. Whereas a previous report found that the composite material containing AuNPs and graphene showed a higher performance in biosensing due to the electrochemical properties,38 our result indicated that the similar composite material did not offer extra benefit in SERS detection compared to AuNPs or graphene alone. R6G is widely used in Raman studies due to its high Raman intensity, so we next examined the ability of our substrate to detect a molecule with weak Raman scattering: BSA. When BSA was deposited on glass, only Raman signals from the glass substrate itself were present in the spectrum (Figure 6A). However, when BSA was deposited on calcined AuNPs (Figure 6A) or on graphene alone (Figures 6B,C), the SERS effect rendered its Raman signals quite strong. Calcination is an important step to generate a clean SERS surface; if polydopamine were not removed, detection of molecules with weak Raman signals would be difficult due to the strong signals of the polydopamine itself and the steric restriction of analyte access to the gold surface where the signal enhancement is highest.
Conclusions We have demonstrated the utility of polydopamine as a functional surface coating. Polydopamine was found to provide a facile method of controlling surface hydrophobicity, and in conjunction with fluorescently labeled tags, polydopamine allowed for the simultaneous and selective detection of specific protein targets. We also demonstrated the ability of polydopamine to sequester a templating array of AuNPs on a surface without the requirement of expensive machinery, such as an electron beam. Sequential deposition of subsequent layers of AuNPs via facile, in situ formation on the deposited surface was achieved with the aid of a mild reductant. Following calcination to remove the polydopamine matrix, the resulting AuNP-coated substrates demonstrated strong SERS enhancement not only of a traditional Raman marker but also of a model protein. Deposition of these arrays was achieved on glass as well as on the SERS-active graphene, demonstrating the adaptability of this technique to various substrate types. Overall, polydopamine seems to hold great potential in the fabrication of functional surface coatings with emergent properties.
Acknowledgement The authors thank Dr. Madhavi Kadakia for reagents and the fluorescent microscope.
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