Polyethyleneglycol diacrylate hydrogels with plasmonic gold nanospheres incorporated via functional group optimization
We present a facile method for the preparation of polyethyleneglycol diacrylate (PEG-DA) hydrogels with plasmonic gold (Au) nanospheres incorporated for various biological and chemical sensing applications. Plasmonic Au nanospheres were prepared ex situ using the standard citrate reduction method with an average diameter of 3.5 nm and a standard deviation of 0.5 nm, and evaluated for their surface functionalization process intended for uniform dispersion in polymer matrices. UV–Visible spectroscopy reveals the existence of plasmonic properties for pristine Au nanospheres, functionalized Au nanospheres, and PEG-DA with uniformly dispersed functionalized Au nanospheres (hybrid Au/PEG-DA hydrogels). Hybrid Au/PEG-DA hydrogels examined by using Fourier transform infra-red spectroscopy (FT-IR) exhibit the characteristic bands at 1635, 1732 and 2882 cm−1 corresponding to reaction products of OH− originating from oxidized product of citrate, –C=O stretching from ester bond, and C–H stretching of PEG-DA, respectively. Thermal studies of hybrid Au/PEG-DA hydrogels show three-stage decomposition with their stabilities up to 500 °C. Optical properties and thermal stabilities associated with the uniform dispersion of Au nanospheres within hydrogels reported herein will facilitate various biological and chemical sensing applications.
KeywordsAu nanospheres Functional groups Hydrogels Plasmonic Polyethyleneglycol diacrylate (PEG-DA)
The field of materials science has been developing steadily over the years, and has today become invaluable in helping us to reach the scientific horizons by providing solutions for bio-medical, sensors, catalysis, pharmaceutical, petrochemical, and mechanical industries, to name a few . An avenue of research that has progressed a great deal in the past few decades in the chemo-sensors via hybrid nanostructured materials by integration of organic, inorganic and biomolecules . Initially, these could only be administered in scant manner, partially due to their limitations in material characterizations and real time applications. Whereas in the current scenario, advancement in analytical instrumentation had made a new paradigm in the development of hybrid materials, which is smart, enough to sense the external stimuli and respond rapidly thus enables the easy access towards hazardous environments . Promising candidates include hydrogel-based hybrid materials, which possess exceptional sensitivity, selectivity, and stability for various external stimuli especially toxic and hazardous gases . As hydrogels replicate the natural systems existing inside the biological organism with three-dimensional (3D) structures, they can be more sensitive towards certain external environments with very high degree of responsivity and selectivity in general. These 3D polymer matrices are capable of imbibing large amounts of water, chemical moieties, large molecules, drugs and biological fluids . The aforementioned properties of hydrogels are the main reasoning behind their diverse applications ranging from sensors, drug delivery, food additives to pharmaceuticals and clinical applications [5, 6]. Synthetic hydrogels provide an effective and controlled way to incur selective target chemical analytes which administer chemo-sensing. The special chemical moieties were used to selectively pick up the hazardous molecules thereby altering their mechanical, optical, electrical, and calorimetric signals which can be read out once integrated with microelectromechanical systems (MEMS) devices . Preparing such smart polymeric hydrogels for sensing applications can be achieved by combining inorganic/organic networks of 3D materials with higher degree of stability obtained via cross-linking. However, the addition of polymer layers with inorganic materials such as Au, platinum (Pt), zinc oxide (ZnO), iron oxide (Fe2O3) etc., can alter the intrinsic properties of the resulting functional inorganic particles . Polymer hydrogel with 3D support should provide enhanced molecular interactions with the functional inorganic materials, thereby creating a hydrophilic environment with more favorable solution kinetics . These kinds of highly oriented molecular interactions were achieved by utilizing the structural property relationship of nanostructured materials in-lieu with their synthetic strategy. It was reported that, anisotropic nanostructures with controlled dimensions and additional functionalities would improve the intrinsic properties of the hybrid materials and make them efficient chemo-sensors [8, 9].
One inorganic material used extensively as chemo-sensors is Au, owing to its high catalytic and sensitization phenomenon . Perhaps, its well-established synthesis procedure for uniform, structural and anisotropic nanostructures with high surface-to-volume ratio makes them well suited for preparing hybrid nanostructures. Integration of polymeric supports with the inorganic nanostructures for preparing functional hybrids mainly depends on the nature of polymer, i.e. chain length, mesh size, etc. . In this context, PEG-DA based materials had shown very promising applications by preparing highly stable hydrogels which were used as mechanical sensors, piezo actuating devices, stimuli response materials and so on . Moreover, polyethyleneglycol diacrylate (PEG-DA) is non-volatile, non-toxic, environmentally benign and tailor made into various shapes, enabling to act as potential stabilizers and matrices for the formation of functional hydrogels . In this paper, a facile method was demonstrated to incorporate functionalized Au nanospheres into PEG-DA polymer matrices by an ex situ approach for the preparation of hybrid Au/PEG-DA hydrogels to further accelerate applications based on PEG-DA and Au hybrids.
Gold chloride (HAuCl4, 99.9%), polyethylene glycol (PEG-200 M n, 99.9%), polyethyleneglycol diacrylate (PEG-DA-700 M n, 95.9%), sodium borohydride (NaBH4, 95.9%), 2-hydroxy-2-methylpropiophenone (99.9%) (Darocur 1173, photo-initiator, PI) and tri sodium citrate (99.9%) were purchased from Aldrich and used for the synthesis without further purification. Lysine (≥98%), thioglycolic acid (≥98%) and glutathione (≥98%) were purchased from Merck Millipore, Korea and used without further purification. Ultra-high pure water (Merck Millipore) with resistivity of 18.2 MΩ-cm was used for the overall synthesis.
Synthesis of Au nanospheres
Au nanospheres were synthesized using the well-established citrate reduction method , where, 20 mL of 2.5 mM of HAuCl4 and tri sodium citrate were taken in a 50 mL round-bottom flask and stirred under ice cold conditions. 0.6 mL of 0.1 M NaBH4 was added as an indicator for the formation of Au nanospheres showing the solution color change from yellow to pink. The synthesized Au nanospheres were purified and washed several times by centrifugation with the ultra-high pure water. Next, the surface of purified Au nanospheres was functionalized with three amino acids [lysine (10%), glutathione (10%) and thioglycolic acid (10%)] individually by magnetic stirring at room temperature. Finally, after 12 h of stirring, the functionalized Au nanospheres solutions were centrifuged and washed several times using the ultra-high pure water to remove the un-reacted amino acids. As it was an ex situ approach, the bonds between Au surface and electron-donating end group of ligand molecules (thiol or amine) undergo dynamic binding and un-binding processes. The ligand molecules bound to the surface of Au nanoparticles by some attractive interactions, such as chemisorption, electrostatic attraction or hydrophobic interactions provided by the head group.
Synthesis of Au/PEG-DA hydrogel
550 µL of functionalized Au nanospheres solution was mixed with 150 µL of PEG in a 1-mL centrifuge tube. Similarly, 250 µL of PEG-DA solution was mixed with 50 µL of PI in another 1-mL centrifuge tube. Then, two solutions mixed separately in different centrifuge tubes were slowly mixed and sonicated for 10 min to achieve uniform dispersion. The final volumetric ratio of each constituent in the Au/PEG/PEG-DA/PI hybrid solution was 55/15/25/5. The prepard mixture was used to fabricate Au/PEG-DA hydrogels by using the UV light emitting diode (CBT-90-ultraviolet-C31-M400-22, Luminus Devices; 8 W and 365 nm) for 3 min to initiate photo-polymerization.
The synthesized materials were characterized using electron microscopy and other analytical techniques. The structure and morphology of the Au nanospheres were examined by using high-resolution transmission electron microscope (HR-TEM) JEOL, JEM-2010 (Japan). UV–Visible absorbance spectra were acquired by using Jasco UV-660 UV–VIS-NIR spectrophotometer (Japan). The FT-IR analysis was performed by using Nicolet Impact 400 FT-IR spectrophotometer with the KBr pellet method. Thermal gravimetric analysis (TGA) was performed by using a STA N-650 simultaneous thermal analyzer (SCINCO) with a heating rate of 10 °C min−1 under Ar (99.999%, 5 N) flow.
Results and discussion
In this study, a facile and rapid chemical method for the uniform dispersion of plasmonic Au nanospheres into the PEG-DA matrix has been developed. Optical and spectroscopic analyses reveal the mono-dispersion of plasmonic nanostructures into the polymer matrix, resulting in the formation of hybrid Au/PEG-DA hydrogels. Au nanospheres with an average diameter of 3.5 ± 0.5 nm and surface plasmon resonance band were confirmed by TEM and UV–Visible spectroscopic studies, respectively. Chemical functionalization of the PEG-DA with functionalized Au nanospheres were observed from the FT-IR studies by the existence of characteristic –C=O stretching and C–H stretching peaks. Thermal stability of the PEG-DA hydrogel had shown significant enhancement after the incorporation of functionalized Au nanospheres owing to the formation of interfacial region and successful dispersion of Au inside the polymer matrix. Thus, functionalized Au/PEG-DA hydrogels would find multiple uses for chemo-sensing applications.
VPD and JL developed the main idea and designed experiments. VPD and SBK performed experiments and data analysis. VPD, SBK and JL drafted the manuscript. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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