Open Access

Polyethyleneglycol diacrylate hydrogels with plasmonic gold nanospheres incorporated via functional group optimization

Micro and Nano Systems Letters20175:21

DOI: 10.1186/s40486-017-0056-8

Received: 21 December 2016

Accepted: 20 April 2017

Published: 27 April 2017

Abstract

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.

Keywords

Au nanospheres Functional groups Hydrogels Plasmonic Polyethyleneglycol diacrylate (PEG-DA)

Background

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 [1]. 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 [2]. 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 [3]. Promising candidates include hydrogel-based hybrid materials, which possess exceptional sensitivity, selectivity, and stability for various external stimuli especially toxic and hazardous gases [4]. 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 [5]. 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 [7]. 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 [8]. 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 [9]. 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 [10]. 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. [11]. 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 [12]. 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 [13]. 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.

Experimental details

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 [14], 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.

Characterization techniques

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

Au nanospheres, synthesized by the standard citrate reduction method [14], and surface functionalization using amino acids to enhance the dispersion ability in the polymer matrix and to avoid agglomeration are schematically shown in Fig. 1. Three functional compounds (lysine, glutathione, and thioglycolic acid) were used for the surface functionalization process.
Fig. 1

Schematic representation of functionalization process for Au nanospheres (NS) using thioglycolic acid (Tga), glutathione reductase (Glu) and lysine (Lys)

Figure 2 shows the TEM images of synthesized Au nanospheres. Highly spherical Au nanospheres with an average diameter of 3.5 ± 0.5 nm (Additional file 1) were observed, depicting the controlled citrate reduction method performed at the ice-cold condition. The HR-TEM image of Au nanospheres (Fig. 2d) reveals the lattice fringes with the inter-planar spacing of 0.236 nm corresponding to (111) plane of face centered cubic (FCC) Au crystal [15].
Fig. 2

a, b TEM images of Au nanospheres, and c, d HR-TEM images of Au nanospheres

Figure 3 represents the UV–Visible spectroscopic studies of pristine and functionalized Au nanospheres. The surface plasmon peak centered at the wavelength of 517 nm was observed for pristine Au nanospheres, ensembles the smaller cluster size of nanospheres as evident from the TEM studies. It was observed that, after functionalization with amino acids such as lysine, glutathione, and thioglycolic acid had made appreciable plasmonic peak shifts (Fig. 3b) owing to the interfacial electron transfer and charge separation processes [16]. As known, Au can accept electrons and mediate the electron transfer process with functional groups thereby generating charge separation and drastic re-arrangement of surface electrons [16, 17]. This process clearly implies the successful immobilization of functional groups over the Au nanospheres surface, which improves the stability of functional nanospheres.
Fig. 3

a UV–Visible spectra and b SPR peak positions of the pristine and functionalized Au (lysine, glutathione, and thioglycolic acid) nanospheres

Thus, functionalized Au nanospheres were mixed with PEG, PEG-DA and PI to prepare the pre-polymer solution as shown in Fig. 4a. The addition of PEG with Au nanospheres and followed by PEG-DA/PI solution ensures the homogenous dispersion of Au in the polymer solution which can be easily identified by varying the PEG-DA and PI ratio from 10 to 35% (Fig. 4b). Among the various ratios prepared, 25% was chosen for further studies with the aid of its high mono-dispersity observed. To obtain functionalized Au/PEG-DA hydrogels as explained in the schematic (Fig. 4a), the selected 25% pre-polymer solution was photo-polymerized by the UV LED (8 W and 365 nm).
Fig. 4

a Schematic illustrating the process functionalized Au/PEG-DA hydrogels and b dispersed Au/PEG/PEG-DA/PI pre-polymer solution, by varying the PEG-DA: PI ratio (10–35%)

The functionalized Au/PEG-DA hydrogels were examined for its surface plasmon resonance (SPR) effect by spectroscopically to confirm the incorporation and bonding of Au nanospheres into the PEG-DA polymer matrix. The functionalized Au/PEG-DA hydrogels showed the characteristic SPR peak, which appears due to the coherent oscillation of the surface conduction electrons as induced by the incident electro-magnetic radiation as shown in Fig. 5. Peak broadening observed for the absorbance maximum in hydrogels due to the presence of acrylic group and dispersion of Au nanospheres, which confirms the interaction of functionalized Au nanospheres with the surrounding polymer matrix [18, 19]. From the UV–Visible spectra, an appreciable red shift (Fig. 5b) was observed for the functionalized Au/PEG-DA hydrogels (lysine, glutathione, and thioglycolic acid) as compared to the pristine Au/PEG-DA hydrogel owing to the interfacial electron transfer process and proving the colloidal stability of functionalized Au nanospheres in the PEG-DA polymer matrix. Thus, functionalization process improved the stability of Au nanospheres inside the PEG-DA, as revealed with the characteristic SPR peak after photo-induced polymerization.
Fig. 5

a UV–Visible spectra and b SPR peak positions of the pristine Au/PEG-DA hydrogel, and functionalized Au/PEG-DA hydrogels (lysine, glutathione, and thioglycolic acid)

Fourier transform infrared spectroscopy with an attenuated total reflection assembly (FT-IR-ATR) mode was employed to study the molecular and structural changes in the hydrogels. Figure 6a shows the FT-IR spectra in the range of 4000–800 cm−1 for the pristine Au/PEG-DA hydrogel and functionalized Au/PEG-DA hydrogels (lysine, glutathione, and thioglycolic acid). The spectrum of the pristine Au/PEG-DA hydrogel (Fig. 6a) shows the absorption bands for acrylic vinyl groups at 810 cm−1 (CH2=CH), 1410 cm−1 (CH2=CH bond) and 1198 cm−1 (C=O) [20]. Similarly, strong peaks at the wavenumber of 1732 and 2882 cm−1 observed correspond to –C=O stretching from ester bonds and C–H stretching vibrations, respectively [21]. The band at 1635 cm−1 corresponds to the reaction product of OH which comes from H2O and oxidized products of citrate produced during the Au3+ reduction [13, 22]. Figure 6b shows that the interactions of Au with PEG-DA were confirmed by presence of intense absorption bands at 1732 and 2882 cm−1 with their difference in intensity after functionalization process and dispersion in the PEG-DA matrix [23]. The FT-IR spectra confirm the molecular structural changes between PEG-DA chains and functionalized Au nanospheres, ensuing by van der Walls interactions inside the polymeric matrix.
Fig. 6

FT-IR spectroscopy of synthesized pristine Au/PEG-DA, lysine functionalized Au/PEG-DA, glutathione functionalized Au/PEG-DA, and thioglycolic acid functionalized Au/PEG-DA in a 4000–800 cm−1 range and b magnified 3300–1600 cm−1 range near peaks from –C=O and C–H bonds of PEG-DA

Thermo-gravimetric studies of PEG-DA hydrogel, pristine Au/PEG-DA hydrogel and functionalized Au/PEG-DA hydrogels (lysine, glutathione, and thioglycolic acid) were shown in Fig. 7a. It was observed that, all materials ensemble the characteristic III stage decomposition. The I stage corresponds to the removal of adsorbed water molecules on the hydrogels surface and the II and III stage decompositions are directly related with the cross-linked PEG and PEG-DA molecules, respectively, formed by the photo-initiation process. Among the five materials, the pristine Au/PEG-DA hydrogel shows lower stability than the functionalized Au/PEG-DA hydrogels, owing to the addition of functional groups and Au nanospheres which favours the cross linking percentage of polymers by chemical bonds and electronic interaction as revealed by FT-IR and UV–Visible studies, respectively [2426]. Thus, functionalized Au/PEG-DA hydrogels can be used for temperature based sensing applications without altering their chemical compositions. Functionalization process of Au nanospheres further improves the dispersion inside the PEG-DA matrix owing to their higher thermal stability. The derivative thermo-gram (Fig. 7b) also reveals the presence of three stage decomposition, corresponding to the removal of water molecule, functional moieties and decomposition of di-acrylate molecules.
Fig. 7

a TGA studies and b DTA studies of synthesized PEG-DA hydrogel, pristine Au/PEG-DA hydrogel, lysine functionalized Au/PEG-DA hydrogel, glutathione functionalized Au/PEG-DA hydrogel, and thioglycolic acid functionalized Au/PEG-DA hydrogel

Conclusions

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.

Declarations

Authors’ contributions

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.

Competing interests

The authors declare that they have no competing interests.

Publisher’s Note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.

Authors’ Affiliations

(1)
Department of Mechanical Engineering, Sogang University

References

  1. Yin PT, Shah S, Chhowalla M, Lee K-B (2015) Design, synthesis, and characterization of graphene–nanoparticle hybrid materials for bio-applications. Chem Rev 115:2483–2531View ArticleGoogle Scholar
  2. Ghosh Chaudhuri R, Paria S (2012) Core/shell nanoparticles: classes, properties, synthesis mechanisms, characterization, and applications. Chem Rev 112:2373–2433View ArticleGoogle Scholar
  3. Cong H-P, Qiu J-H, Yu S-H (2015) Thermoresponsive Poly(N-isopropylacrylamide)/Graphene/Au nanocomposite hydrogel for water treatment by a laser-assisted approach. Small 11:1165–1170View ArticleGoogle Scholar
  4. Hood M, Mari M, Muñoz-Espí R (2014) Synthetic strategies in the preparation of polymer/inorganic hybrid nanoparticles. Materials 7:4057View ArticleGoogle Scholar
  5. Buenger D, Topuz F, Groll J (2012) Hydrogels in sensing applications. Prog Polym Sci 37:1678–1719View ArticleGoogle Scholar
  6. Ahmed EM (2015) Hydrogel: preparation, characterization, and applications: a review. J Adv Res 6:105–121View ArticleGoogle Scholar
  7. Oyen ML (2014) Mechanical characterisation of hydrogel materials. Int Mater Rev 59:44–59View ArticleGoogle Scholar
  8. Koo W-T, Choi S-J, Kim S-J, Jang J-S, Tuller HL, Kim I-D (2016) Heterogeneous sensitization of metal–organic framework driven metal@metal oxide complex catalysts on an oxide nanofiber scaffold toward superior gas sensors. J Am Chem Soc 138:13431–13437View ArticleGoogle Scholar
  9. Potyrailo RA (2016) Multivariable sensors for ubiquitous monitoring of gases in the era of internet of things and industrial internet. Chem Rev 116:11877–11923View ArticleGoogle Scholar
  10. Zeng S, Yong K-T, Roy I, Dinh X-Q, Yu X, Luan F (2011) A review on functionalized gold nanoparticles for biosensing applications. Plasmonics 6:491–506View ArticleGoogle Scholar
  11. Cong H-P, Qiu J-H, Yu S-H (2015) Thermoresponsive poly(n-isopropylacrylamide)/graphene/au nanocomposite hydrogel for water treatment by a laser-assisted approach. Small 11:1165–1170View ArticleGoogle Scholar
  12. Lee JS, Song J, Kim SO, Kim S, Lee W, Jackman JA, Kim D, Cho N-J, Lee J (2016) Multifunctional hydrogel nano-probes for atomic force microscopy. Nat Commun 7:11566View ArticleGoogle Scholar
  13. Fang H, Jiang F, Wu Q, Ding Y, Wang Z (2014) Supertough polylactide materials prepared through in situ reactive blending with peg-based diacrylate monomer. ACS Appl Mater Interfaces 6:13552–13563View ArticleGoogle Scholar
  14. Jana NR, Gearheart L, Murphy CJ (2001) Seed-mediated growth approach for shape-controlled synthesis of spheroidal and rod-like gold nanoparticles using a surfactant template. Adv Mater 13:1389–1393View ArticleGoogle Scholar
  15. Tan N, Lee C, Li P (2016) Green synthesis of smart metal/polymer nanocomposite particles and their tuneable catalytic activities. Polymers 8:105View ArticleGoogle Scholar
  16. Selvakannan PR, Mandal S, Phadtare S, Pasricha R, Sastry M (2003) Capping of gold nanoparticles by the amino acid lysine renders them water-dispersible. Langmuir 19:3545–3549View ArticleGoogle Scholar
  17. Shankar SS, Bhargava S, Sastry M (2005) Synthesis of gold nanospheres and nanotriangles by the turkevich approach. J Nanosci Nanotechnol 5:1721–1727View ArticleGoogle Scholar
  18. Fang L, Li Y, Chen Z, Liu W, Zhang J, Xiang S, Shen H, Li Z, Yang B (2014) Tunable polymer brush/au nps hybrid plasmonic arrays based on host–guest interaction. ACS Appl Mater Interfaces 6:19951–19957View ArticleGoogle Scholar
  19. Amici J, Sangermano M, Celasco E, Yagci Y (2011) Photochemical synthesis of gold–polyethylenglycol core–shell nanoparticles. Eur Polym J 47:1250–1255View ArticleGoogle Scholar
  20. Shameli K, Bin Ahmad M, Jazayeri SD, Sedaghat S, Shabanzadeh P, Jahangirian H, Mahdavi M, Abdollahi Y (2012) Synthesis and characterization of polyethylene glycol mediated silver nanoparticles by the green method. Int J Mol Sci 13:6639View ArticleGoogle Scholar
  21. Kunst SR, Beltrami LVR, Cardoso HRP, Veja MRO, Baldin EKK, Menezes TL, Malfatti CdF (2014) Effect of curing temperature and architectural (monolayer and bilayer) of hybrid films modified with polyethylene glycol for the corrosion protection on tinplate. Mater Res 17:1071–1081View ArticleGoogle Scholar
  22. Narayan S, Rajagopalan A, Reddy JS, Chadha A (2014) BSA binding to silica capped gold nanostructures: effect of surface cap and conjugation design on nanostructure-BSA interface. RSC Adv 4:1412–1420View ArticleGoogle Scholar
  23. Xiao Y, He L, Che J (2012) An effective approach for the fabrication of reinforced composite hydrogel engineered with SWNTs, polypyrrole and PEGDA hydrogel. J Mater Chem 22:8076–8082View ArticleGoogle Scholar
  24. Zhong C, Chu CC (2012) Biomimetic mineralization of acid polysaccharide-based hydrogels: towards porous 3-dimensional bone-like biocomposites. J Mater Chem 22:6080–6087View ArticleGoogle Scholar
  25. Liu H, Qian X, Wu Z, Yang R, Sun S, Ma H (2016) Microfluidic synthesis of QD-encoded PEGDA microspheres for suspension assay. J Mater Chem B 4:482–488View ArticleGoogle Scholar
  26. He H, Adzima B, Zhong M, Averick S, Koepsel R, Murata H, Russell A, Luebke D, Takahara A, Nulwala H, Matyjaszewski K (2014) Multifunctional photo-cross linked polymeric ionic hydrogel films. Polym Chem 5:2824–2835View ArticleGoogle Scholar

Copyright

© The Author(s) 2017