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

Polyethyleneglycol diacrylate hydrogels with plasmonic gold nanospheres incorporated via... 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 nano- spheres 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 spec- −1 troscopy (FT-IR) exhibit the characteristic bands at 1635, 1732 and 2882 cm 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 and respond rapidly thus enables the easy access towards The field of materials science has been developing stead - hazardous environments [3]. Promising candidates ily over the years, and has today become invaluable in include hydrogel-based hybrid materials, which possess helping us to reach the scientific horizons by providing exceptional sensitivity, selectivity, and stability for vari- solutions for bio-medical, sensors, catalysis, pharmaceu- ous external stimuli especially toxic and hazardous gases tical, petrochemical, and mechanical industries, to name [4]. As hydrogels replicate the natural systems existing a few [1]. An avenue of research that has progressed a inside the biological organism with three-dimensional great deal in the past few decades in the chemo-sensors (3D) structures, they can be more sensitive towards via hybrid nanostructured materials by integration of certain external environments with very high degree of organic, inorganic and biomolecules [2]. Initially, these responsivity and selectivity in general. These 3D polymer could only be administered in scant manner, partially matrices are capable of imbibing large amounts of water, due to their limitations in material characterizations and chemical moieties, large molecules, drugs and biological real time applications. Whereas in the current scenario, fluids [ 5]. The aforementioned properties of hydrogels advancement in analytical instrumentation had made are the main reasoning behind their diverse applications a new paradigm in the development of hybrid materi- ranging from sensors, drug delivery, food additives to als, which is smart, enough to sense the external stimuli pharmaceuticals and clinical applications [5, 6]. Synthetic hydrogels provide an effective and controlled way to incur selective target chemical analytes which administer *Correspondence: jayclee@sogang.ac.kr chemo-sensing. The special chemical moieties were used Department of Mechanical Engineering, Sogang University, 35 Baekbeom-ro (Sinsu-dong), Mapo-gu, Seoul 04107, South Korea © The Author(s) 2017. This 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. Ponnuvelu et al. Micro and Nano Syst Lett (2017) 5:21 Page 2 of 8 to selectively pick up the hazardous molecules thereby (99.9%) were purchased from Aldrich and used for the altering their mechanical, optical, electrical, and calo- synthesis without further purification. Lysine (≥98%), rimetric signals which can be read out once integrated thioglycolic acid (≥98%) and glutathione (≥98%) were with microelectromechanical systems (MEMS) devices purchased from Merck Millipore, Korea and used with- [7]. Preparing such smart polymeric hydrogels for sens- out further purification. Ultra-high pure water (Merck ing applications can be achieved by combining inorganic/ Millipore) with resistivity of 18.2  MΩ-cm was used for organic networks of 3D materials with higher degree of the overall synthesis. stability obtained via cross-linking. However, the addi- tion of polymer layers with inorganic materials such as Synthesis of Au nanospheres Au, platinum (Pt), zinc oxide (ZnO), iron oxide (F e O ) Au nanospheres were synthesized using the well-estab- 2 3 etc., can alter the intrinsic properties of the resulting lished citrate reduction method [14], where, 20  mL of functional inorganic particles [8]. Polymer hydrogel with 2.5  mM of H AuCl and tri sodium citrate were taken in 3D support should provide enhanced molecular inter- a 50  mL round-bottom flask and stirred under ice cold actions with the functional inorganic materials, thereby conditions.  0.6  mL of 0.1  M NaBH was added as an creating a hydrophilic environment with more favora- indicator for the formation of Au nanospheres show- ble solution kinetics [9]. These kinds of highly oriented ing the solution color change from yellow to pink. The molecular interactions were achieved by utilizing the synthesized Au nanospheres were purified and washed structural property relationship of nanostructured mate- several times by centrifugation with the ultra-high pure rials in-lieu with their synthetic strategy. It was reported water. Next, the surface of purified Au nanospheres was that, anisotropic nanostructures with controlled dimen- functionalized with three amino acids [lysine (10%), glu- sions and additional functionalities would improve the tathione (10%) and thioglycolic acid (10%)] individually intrinsic properties of the hybrid materials and make by magnetic stirring at room temperature. Finally, after them efficient chemo-sensors [8, 9]. 12 h of stirring, the functionalized Au nanospheres solu- One inorganic material used extensively as chemo- tions were centrifuged and washed several times using sensors is Au, owing to its high catalytic and sensitization the ultra-high pure water to remove the un-reacted phenomenon [10]. Perhaps, its well-established synthesis amino acids. As it was an ex situ approach, the bonds procedure for uniform, structural and anisotropic nano- between Au surface and electron-donating end group structures with high surface-to-volume ratio makes them of ligand molecules (thiol or amine) undergo dynamic well suited for preparing hybrid nanostructures. Integra- binding and un-binding processes. The ligand molecules tion of polymeric supports with the inorganic nanostruc- bound to the surface of Au nanoparticles by some attrac- tures for preparing functional hybrids mainly depends tive interactions, such as chemisorption, electrostatic on the nature of polymer, i.e. chain length, mesh size, attraction or hydrophobic interactions provided by the etc. [11]. In this context, PEG-DA based materials had head group. shown very promising applications by preparing highly stable hydrogels which were used as mechanical sen- Synthesis of Au/PEG‑DA hydrogel sors, piezo actuating devices, stimuli response materials 550 µL of functionalized Au nanospheres solution was and so on [12]. Moreover, polyethyleneglycol diacrylate mixed with 150 µL of PEG in a 1-mL centrifuge tube. (PEG-DA) is non-volatile, non-toxic, environmentally Similarly, 250 µL of PEG-DA solution was mixed with benign and tailor made into various shapes, enabling to 50 µL of PI in another 1-mL centrifuge tube. Then, two act as potential stabilizers and matrices for the forma- solutions mixed separately in different centrifuge tubes tion of functional hydrogels [13]. In this paper, a facile were slowly mixed and sonicated for 10  min to achieve method was demonstrated to incorporate functionalized uniform dispersion. The final volumetric ratio of each Au nanospheres into PEG-DA polymer matrices by an constituent in the Au/PEG/PEG-DA/PI hybrid solu- ex situ approach for the preparation of hybrid Au/PEG- tion was 55/15/25/5. The prepard mixture was used to DA hydrogels to further accelerate applications based on fabricate Au/PEG-DA hydrogels by using the UV light PEG-DA and Au hybrids. emitting diode (CBT-90-ultraviolet-C31-M400-22, Luminus Devices; 8 W and 365 nm) for 3 min to initiate Experimental details photo-polymerization. Gold chloride (HAuCl , 99.9%), polyethylene glycol (PEG-200 M , 99.9%), polyethyleneglycol diacrylate Characterization techniques (PEG-DA-700 M , 95.9%), sodium borohydride (N aBH , The synthesized materials were characterized using elec - n 4 95.9%), 2-hydroxy-2-methylpropiophenone (99.9%) tron microscopy and other analytical techniques. The (Darocur 1173, photo-initiator, PI) and tri sodium citrate structure and morphology of the Au nanospheres were Ponnuvelu et al. Micro and Nano Syst Lett (2017) 5:21 Page 3 of 8 examined by using high-resolution transmission elec- corresponding to (111) plane of face centered cubic tron microscope (HR-TEM) JEOL, JEM-2010 (Japan). (FCC) Au crystal [15]. UV–Visible absorbance spectra were acquired by using Figure 3 represents the UV–Visible spectroscopic studies Jasco UV-660 UV–VIS-NIR spectrophotometer (Japan). of pristine and functionalized Au nanospheres. The sur - The FT-IR analysis was performed by using Nicolet face plasmon peak centered at the wavelength of 517  nm Impact 400 FT-IR spectrophotometer with the KBr pel- was observed for pristine Au nanospheres, ensembles let method. Thermal gravimetric analysis (TGA) was the smaller cluster size of nanospheres as evident from performed by using a STA N-650 simultaneous thermal the TEM studies. It was observed that, after function- −1 analyzer (SCINCO) with a heating rate of 10  °C min alization with amino acids such as lysine, glutathione, and under Ar (99.999%, 5 N) flow. thioglycolic acid had made appreciable plasmonic peak shifts (Fig.  3b) owing to the interfacial electron transfer Results and discussion and charge separation processes [16]. As known, Au can Au nanospheres, synthesized by the standard citrate accept electrons and mediate the electron transfer process reduction method [14], and surface functionalization with functional groups thereby generating charge separa- using amino acids to enhance the dispersion ability in tion and drastic re-arrangement of surface electrons [16, the polymer matrix and to avoid agglomeration are sche- 17]. This process clearly implies the successful immobiliza - matically shown in Fig.  1. Three functional compounds tion of functional groups over the Au nanospheres surface, (lysine, glutathione, and thioglycolic acid) were used for which improves the stability of functional nanospheres. the surface functionalization process. uTh s, functionalized Au nanospheres were mixed Figure  2 shows the TEM images of synthesized Au with PEG, PEG-DA and PI to prepare the pre-polymer nanospheres. Highly spherical Au nanospheres with solution as shown in Fig.  4a. The addition of PEG with an average diameter of 3.5  ±  0.5  nm (Additional file  1) Au nanospheres and followed by PEG-DA/PI solution were observed, depicting the controlled citrate reduc- ensures the homogenous dispersion of Au in the poly- tion method performed at the ice-cold condition. The mer solution which can be easily identified by varying the HR-TEM image of Au nanospheres (Fig.  2d) reveals the PEG-DA and PI ratio from 10 to 35% (Fig.  4b). Among lattice fringes with the inter-planar spacing of 0.236  nm the various ratios prepared, 25% was chosen for further Fig. 1 Schematic representation of functionalization process for Au nanospheres (NS) using thioglycolic acid ( Tga), glutathione reductase (Glu) and lysine (Lys) Ponnuvelu et al. Micro and Nano Syst Lett (2017) 5:21 Page 4 of 8 Fig. 2 a, b TEM images of Au nanospheres, and c, d HR-TEM images of Au nanospheres Fig. 3 a UV–Visible spectra and b SPR peak positions of the pristine and functionalized Au (lysine, glutathione, and thioglycolic acid) nanospheres Ponnuvelu et al. Micro and Nano Syst Lett (2017) 5:21 Page 5 of 8 Fig. 4 a Schematic illustrating the process functionalized Au/PEG-DA hydrogels and b dispersed Au/PEG/PEG-DA/PI pre-polymer solution, by vary- ing the PEG-DA: PI ratio (10–35%) studies with the aid of its high mono-dispersity observed. to the coherent oscillation of the surface conduction elec- To obtain functionalized Au/PEG-DA hydrogels as trons as induced by the incident electro-magnetic radia- explained in the schematic (Fig.  4a), the selected 25% tion as shown in Fig. 5. Peak broadening observed for the pre-polymer solution was photo-polymerized by the UV absorbance maximum in hydrogels due to the presence LED (8 W and 365 nm). of acrylic group and dispersion of Au nanospheres, which The functionalized Au/PEG-DA hydrogels were exam - confirms the interaction of functionalized Au nano - ined for its surface plasmon resonance (SPR) effect by spheres with the surrounding polymer matrix [18, 19]. spectroscopically to confirm the incorporation and From the UV–Visible spectra, an appreciable red shift bonding of Au nanospheres into the PEG-DA poly- (Fig.  5b) was observed for the functionalized Au/PEG- mer matrix. The functionalized  Au/PEG-DA hydrogels DA hydrogels (lysine, glutathione, and thioglycolic acid) showed the characteristic SPR peak, which appears due as compared to the pristine Au/PEG-DA hydrogel owing 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, glu- tathione, and thioglycolic acid) Ponnuvelu et al. Micro and Nano Syst Lett (2017) 5:21 Page 6 of 8 to the interfacial electron transfer process and proving Thermo-gravimetric studies of PEG-DA hydrogel, pris - the colloidal stability of functionalized Au nanospheres tine Au/PEG-DA hydrogel and functionalized Au/PEG- in the PEG-DA polymer matrix. Thus, functionalization DA hydrogels (lysine, glutathione, and thioglycolic acid) process improved the stability of Au nanospheres inside were shown in Fig.  7a. It was observed that, all materi- the PEG-DA, as revealed with the characteristic SPR als ensemble the characteristic III stage decomposition. peak after photo-induced polymerization. The I stage corresponds to the removal of adsorbed water Fourier transform infrared spectroscopy with an attenu- molecules on the hydrogels surface  and the II and III ated total reflection assembly (FT-IR-ATR) mode was stage decompositions are directly related with the cross- employed to study the molecular and structural changes linked PEG and PEG-DA molecules, respectively, formed in the hydrogels. Figure  6a shows the FT-IR spectra in by the photo-initiation process. Among the five materi - −1 the range of 4000–800  cm for the pristine Au/PEG- als, the pristine Au/PEG-DA hydrogel shows lower stabil- DA hydrogel and functionalized Au/PEG-DA hydrogels ity than the functionalized Au/PEG-DA hydrogels, owing (lysine, glutathione, and thioglycolic acid). The spectrum to the addition of functional groups and Au nanospheres of the pristine Au/PEG-DA hydrogel (Fig.  6a) shows the which favours the cross linking percentage of polymers −1 absorption bands for acrylic vinyl groups at 810  cm by chemical bonds and electronic interaction as revealed −1 −1 (CH =CH), 1410  cm (CH =CH bond) and 1198  cm by FT-IR and UV–Visible studies, respectively [24–26]. 2 2 (C=O) [20]. Similarly, strong peaks at the wavenumber u Th s, functionalized Au/PEG-DA hydrogels can be used −1 of 1732 and 2882  cm observed correspond to –C=O for temperature based sensing applications without alter- stretching from ester bonds and C–H stretching vibra- ing their chemical compositions. Functionalization pro- −1 tions, respectively [21]. The band at 1635  cm corre- cess of Au nanospheres further improves the dispersion sponds to the reaction product of OH which comes inside the PEG-DA matrix owing to their higher ther- from H O and oxidized products of citrate produced dur- mal stability. The derivative thermo-gram (Fig.  7b) also 3+ ing the Au reduction [13, 22]. Figure 6b shows that the reveals the presence of three stage decomposition, cor- interactions of Au with PEG-DA were confirmed by pres - responding to the removal of water molecule, functional −1 ence of intense absorption bands at 1732 and 2882 cm moieties and decomposition of di-acrylate molecules. with their difference in intensity after functionalization process and dispersion in the PEG-DA matrix [23]. The Conclusions FT-IR spectra confirm the molecular structural changes In this study, a facile and rapid chemical method for between PEG-DA chains and functionalized Au nano- the uniform dispersion of plasmonic Au nanospheres spheres, ensuing by van der Walls interactions inside the into the PEG-DA matrix has been developed. Optical polymeric matrix. and spectroscopic analyses reveal the mono-dispersion Fig. 6 FT-IR spectroscopy of synthesized pristine Au/PEG-DA, lysine functionalized Au/PEG-DA, glutathione functionalized Au/PEG-DA, and thiogly- −1 −1 colic acid functionalized Au/PEG-DA in a 4000–800 cm range and b magnified 3300–1600 cm range near peaks from –C=O and C–H bonds of PEG-DA Ponnuvelu et al. Micro and Nano Syst Lett (2017) 5:21 Page 7 of 8 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 Received: 21 December 2016 Accepted: 20 April 2017 of plasmonic nanostructures into the polymer matrix, resulting in the formation of hybrid Au/PEG-DA hydro- gels. 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 stud - References 1. Yin PT, Shah S, Chhowalla M, Lee K-B (2015) Design, synthesis, and charac- ies, respectively. Chemical functionalization of the PEG- terization of graphene–nanoparticle hybrid materials for bio-applications. DA with functionalized Au nanospheres were observed Chem Rev 115:2483–2531 from the FT-IR studies by the existence of characteristic 2. 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Ahmed EM (2015) Hydrogel: preparation, characterization, and applica- tions: a review. J Adv Res 6:105–121 7. Oyen ML (2014) Mechanical characterisation of hydrogel materials. Int Additional file Mater Rev 59:44–59 8. Koo W-T, Choi S-J, Kim S-J, Jang J-S, Tuller HL, Kim I-D (2016) Heterogene- ous sensitization of metal–organic framework driven metal@metal oxide Additional file 1: Figure S1. Size histogram for synthesized gold nano - complex catalysts on an oxide nanofiber scaffold toward superior gas spheres characterized by TEM. sensors. J Am Chem Soc 138:13431–13437 9. Potyrailo RA (2016) Multivariable sensors for ubiquitous monitoring of gases in the era of internet of things and industrial internet. Chem Rev 116:11877–11923 Authors’ contributions 10. Zeng S, Yong K-T, Roy I, Dinh X-Q, Yu X, Luan F (2011) A review on func- VPD and JL developed the main idea and designed experiments. VPD and tionalized gold nanoparticles for biosensing applications. 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Polyethyleneglycol diacrylate hydrogels with plasmonic gold nanospheres incorporated via functional group optimization

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Springer Journals
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Copyright © 2017 by The Author(s)
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Engineering; Circuits and Systems; Electrical Engineering; Mechanical Engineering; Nanotechnology; Applied and Technical Physics
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2213-9621
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10.1186/s40486-017-0056-8
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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 nano- spheres 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 spec- −1 troscopy (FT-IR) exhibit the characteristic bands at 1635, 1732 and 2882 cm 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 and respond rapidly thus enables the easy access towards The field of materials science has been developing stead - hazardous environments [3]. Promising candidates ily over the years, and has today become invaluable in include hydrogel-based hybrid materials, which possess helping us to reach the scientific horizons by providing exceptional sensitivity, selectivity, and stability for vari- solutions for bio-medical, sensors, catalysis, pharmaceu- ous external stimuli especially toxic and hazardous gases tical, petrochemical, and mechanical industries, to name [4]. As hydrogels replicate the natural systems existing a few [1]. An avenue of research that has progressed a inside the biological organism with three-dimensional great deal in the past few decades in the chemo-sensors (3D) structures, they can be more sensitive towards via hybrid nanostructured materials by integration of certain external environments with very high degree of organic, inorganic and biomolecules [2]. Initially, these responsivity and selectivity in general. These 3D polymer could only be administered in scant manner, partially matrices are capable of imbibing large amounts of water, due to their limitations in material characterizations and chemical moieties, large molecules, drugs and biological real time applications. Whereas in the current scenario, fluids [ 5]. The aforementioned properties of hydrogels advancement in analytical instrumentation had made are the main reasoning behind their diverse applications a new paradigm in the development of hybrid materi- ranging from sensors, drug delivery, food additives to als, which is smart, enough to sense the external stimuli pharmaceuticals and clinical applications [5, 6]. Synthetic hydrogels provide an effective and controlled way to incur selective target chemical analytes which administer *Correspondence: jayclee@sogang.ac.kr chemo-sensing. The special chemical moieties were used Department of Mechanical Engineering, Sogang University, 35 Baekbeom-ro (Sinsu-dong), Mapo-gu, Seoul 04107, South Korea © The Author(s) 2017. This 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. Ponnuvelu et al. Micro and Nano Syst Lett (2017) 5:21 Page 2 of 8 to selectively pick up the hazardous molecules thereby (99.9%) were purchased from Aldrich and used for the altering their mechanical, optical, electrical, and calo- synthesis without further purification. Lysine (≥98%), rimetric signals which can be read out once integrated thioglycolic acid (≥98%) and glutathione (≥98%) were with microelectromechanical systems (MEMS) devices purchased from Merck Millipore, Korea and used with- [7]. Preparing such smart polymeric hydrogels for sens- out further purification. Ultra-high pure water (Merck ing applications can be achieved by combining inorganic/ Millipore) with resistivity of 18.2  MΩ-cm was used for organic networks of 3D materials with higher degree of the overall synthesis. stability obtained via cross-linking. However, the addi- tion of polymer layers with inorganic materials such as Synthesis of Au nanospheres Au, platinum (Pt), zinc oxide (ZnO), iron oxide (F e O ) Au nanospheres were synthesized using the well-estab- 2 3 etc., can alter the intrinsic properties of the resulting lished citrate reduction method [14], where, 20  mL of functional inorganic particles [8]. Polymer hydrogel with 2.5  mM of H AuCl and tri sodium citrate were taken in 3D support should provide enhanced molecular inter- a 50  mL round-bottom flask and stirred under ice cold actions with the functional inorganic materials, thereby conditions.  0.6  mL of 0.1  M NaBH was added as an creating a hydrophilic environment with more favora- indicator for the formation of Au nanospheres show- ble solution kinetics [9]. These kinds of highly oriented ing the solution color change from yellow to pink. The molecular interactions were achieved by utilizing the synthesized Au nanospheres were purified and washed structural property relationship of nanostructured mate- several times by centrifugation with the ultra-high pure rials in-lieu with their synthetic strategy. It was reported water. Next, the surface of purified Au nanospheres was that, anisotropic nanostructures with controlled dimen- functionalized with three amino acids [lysine (10%), glu- sions and additional functionalities would improve the tathione (10%) and thioglycolic acid (10%)] individually intrinsic properties of the hybrid materials and make by magnetic stirring at room temperature. Finally, after them efficient chemo-sensors [8, 9]. 12 h of stirring, the functionalized Au nanospheres solu- One inorganic material used extensively as chemo- tions were centrifuged and washed several times using sensors is Au, owing to its high catalytic and sensitization the ultra-high pure water to remove the un-reacted phenomenon [10]. Perhaps, its well-established synthesis amino acids. As it was an ex situ approach, the bonds procedure for uniform, structural and anisotropic nano- between Au surface and electron-donating end group structures with high surface-to-volume ratio makes them of ligand molecules (thiol or amine) undergo dynamic well suited for preparing hybrid nanostructures. Integra- binding and un-binding processes. The ligand molecules tion of polymeric supports with the inorganic nanostruc- bound to the surface of Au nanoparticles by some attrac- tures for preparing functional hybrids mainly depends tive interactions, such as chemisorption, electrostatic on the nature of polymer, i.e. chain length, mesh size, attraction or hydrophobic interactions provided by the etc. [11]. In this context, PEG-DA based materials had head group. shown very promising applications by preparing highly stable hydrogels which were used as mechanical sen- Synthesis of Au/PEG‑DA hydrogel sors, piezo actuating devices, stimuli response materials 550 µL of functionalized Au nanospheres solution was and so on [12]. Moreover, polyethyleneglycol diacrylate mixed with 150 µL of PEG in a 1-mL centrifuge tube. (PEG-DA) is non-volatile, non-toxic, environmentally Similarly, 250 µL of PEG-DA solution was mixed with benign and tailor made into various shapes, enabling to 50 µL of PI in another 1-mL centrifuge tube. Then, two act as potential stabilizers and matrices for the forma- solutions mixed separately in different centrifuge tubes tion of functional hydrogels [13]. In this paper, a facile were slowly mixed and sonicated for 10  min to achieve method was demonstrated to incorporate functionalized uniform dispersion. The final volumetric ratio of each Au nanospheres into PEG-DA polymer matrices by an constituent in the Au/PEG/PEG-DA/PI hybrid solu- ex situ approach for the preparation of hybrid Au/PEG- tion was 55/15/25/5. The prepard mixture was used to DA hydrogels to further accelerate applications based on fabricate Au/PEG-DA hydrogels by using the UV light PEG-DA and Au hybrids. emitting diode (CBT-90-ultraviolet-C31-M400-22, Luminus Devices; 8 W and 365 nm) for 3 min to initiate Experimental details photo-polymerization. Gold chloride (HAuCl , 99.9%), polyethylene glycol (PEG-200 M , 99.9%), polyethyleneglycol diacrylate Characterization techniques (PEG-DA-700 M , 95.9%), sodium borohydride (N aBH , The synthesized materials were characterized using elec - n 4 95.9%), 2-hydroxy-2-methylpropiophenone (99.9%) tron microscopy and other analytical techniques. The (Darocur 1173, photo-initiator, PI) and tri sodium citrate structure and morphology of the Au nanospheres were Ponnuvelu et al. Micro and Nano Syst Lett (2017) 5:21 Page 3 of 8 examined by using high-resolution transmission elec- corresponding to (111) plane of face centered cubic tron microscope (HR-TEM) JEOL, JEM-2010 (Japan). (FCC) Au crystal [15]. UV–Visible absorbance spectra were acquired by using Figure 3 represents the UV–Visible spectroscopic studies Jasco UV-660 UV–VIS-NIR spectrophotometer (Japan). of pristine and functionalized Au nanospheres. The sur - The FT-IR analysis was performed by using Nicolet face plasmon peak centered at the wavelength of 517  nm Impact 400 FT-IR spectrophotometer with the KBr pel- was observed for pristine Au nanospheres, ensembles let method. Thermal gravimetric analysis (TGA) was the smaller cluster size of nanospheres as evident from performed by using a STA N-650 simultaneous thermal the TEM studies. It was observed that, after function- −1 analyzer (SCINCO) with a heating rate of 10  °C min alization with amino acids such as lysine, glutathione, and under Ar (99.999%, 5 N) flow. thioglycolic acid had made appreciable plasmonic peak shifts (Fig.  3b) owing to the interfacial electron transfer Results and discussion and charge separation processes [16]. As known, Au can Au nanospheres, synthesized by the standard citrate accept electrons and mediate the electron transfer process reduction method [14], and surface functionalization with functional groups thereby generating charge separa- using amino acids to enhance the dispersion ability in tion and drastic re-arrangement of surface electrons [16, the polymer matrix and to avoid agglomeration are sche- 17]. This process clearly implies the successful immobiliza - matically shown in Fig.  1. Three functional compounds tion of functional groups over the Au nanospheres surface, (lysine, glutathione, and thioglycolic acid) were used for which improves the stability of functional nanospheres. the surface functionalization process. uTh s, functionalized Au nanospheres were mixed Figure  2 shows the TEM images of synthesized Au with PEG, PEG-DA and PI to prepare the pre-polymer nanospheres. Highly spherical Au nanospheres with solution as shown in Fig.  4a. The addition of PEG with an average diameter of 3.5  ±  0.5  nm (Additional file  1) Au nanospheres and followed by PEG-DA/PI solution were observed, depicting the controlled citrate reduc- ensures the homogenous dispersion of Au in the poly- tion method performed at the ice-cold condition. The mer solution which can be easily identified by varying the HR-TEM image of Au nanospheres (Fig.  2d) reveals the PEG-DA and PI ratio from 10 to 35% (Fig.  4b). Among lattice fringes with the inter-planar spacing of 0.236  nm the various ratios prepared, 25% was chosen for further Fig. 1 Schematic representation of functionalization process for Au nanospheres (NS) using thioglycolic acid ( Tga), glutathione reductase (Glu) and lysine (Lys) Ponnuvelu et al. Micro and Nano Syst Lett (2017) 5:21 Page 4 of 8 Fig. 2 a, b TEM images of Au nanospheres, and c, d HR-TEM images of Au nanospheres Fig. 3 a UV–Visible spectra and b SPR peak positions of the pristine and functionalized Au (lysine, glutathione, and thioglycolic acid) nanospheres Ponnuvelu et al. Micro and Nano Syst Lett (2017) 5:21 Page 5 of 8 Fig. 4 a Schematic illustrating the process functionalized Au/PEG-DA hydrogels and b dispersed Au/PEG/PEG-DA/PI pre-polymer solution, by vary- ing the PEG-DA: PI ratio (10–35%) studies with the aid of its high mono-dispersity observed. to the coherent oscillation of the surface conduction elec- To obtain functionalized Au/PEG-DA hydrogels as trons as induced by the incident electro-magnetic radia- explained in the schematic (Fig.  4a), the selected 25% tion as shown in Fig. 5. Peak broadening observed for the pre-polymer solution was photo-polymerized by the UV absorbance maximum in hydrogels due to the presence LED (8 W and 365 nm). of acrylic group and dispersion of Au nanospheres, which The functionalized Au/PEG-DA hydrogels were exam - confirms the interaction of functionalized Au nano - ined for its surface plasmon resonance (SPR) effect by spheres with the surrounding polymer matrix [18, 19]. spectroscopically to confirm the incorporation and From the UV–Visible spectra, an appreciable red shift bonding of Au nanospheres into the PEG-DA poly- (Fig.  5b) was observed for the functionalized Au/PEG- mer matrix. The functionalized  Au/PEG-DA hydrogels DA hydrogels (lysine, glutathione, and thioglycolic acid) showed the characteristic SPR peak, which appears due as compared to the pristine Au/PEG-DA hydrogel owing 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, glu- tathione, and thioglycolic acid) Ponnuvelu et al. Micro and Nano Syst Lett (2017) 5:21 Page 6 of 8 to the interfacial electron transfer process and proving Thermo-gravimetric studies of PEG-DA hydrogel, pris - the colloidal stability of functionalized Au nanospheres tine Au/PEG-DA hydrogel and functionalized Au/PEG- in the PEG-DA polymer matrix. Thus, functionalization DA hydrogels (lysine, glutathione, and thioglycolic acid) process improved the stability of Au nanospheres inside were shown in Fig.  7a. It was observed that, all materi- the PEG-DA, as revealed with the characteristic SPR als ensemble the characteristic III stage decomposition. peak after photo-induced polymerization. The I stage corresponds to the removal of adsorbed water Fourier transform infrared spectroscopy with an attenu- molecules on the hydrogels surface  and the II and III ated total reflection assembly (FT-IR-ATR) mode was stage decompositions are directly related with the cross- employed to study the molecular and structural changes linked PEG and PEG-DA molecules, respectively, formed in the hydrogels. Figure  6a shows the FT-IR spectra in by the photo-initiation process. Among the five materi - −1 the range of 4000–800  cm for the pristine Au/PEG- als, the pristine Au/PEG-DA hydrogel shows lower stabil- DA hydrogel and functionalized Au/PEG-DA hydrogels ity than the functionalized Au/PEG-DA hydrogels, owing (lysine, glutathione, and thioglycolic acid). The spectrum to the addition of functional groups and Au nanospheres of the pristine Au/PEG-DA hydrogel (Fig.  6a) shows the which favours the cross linking percentage of polymers −1 absorption bands for acrylic vinyl groups at 810  cm by chemical bonds and electronic interaction as revealed −1 −1 (CH =CH), 1410  cm (CH =CH bond) and 1198  cm by FT-IR and UV–Visible studies, respectively [24–26]. 2 2 (C=O) [20]. Similarly, strong peaks at the wavenumber u Th s, functionalized Au/PEG-DA hydrogels can be used −1 of 1732 and 2882  cm observed correspond to –C=O for temperature based sensing applications without alter- stretching from ester bonds and C–H stretching vibra- ing their chemical compositions. Functionalization pro- −1 tions, respectively [21]. The band at 1635  cm corre- cess of Au nanospheres further improves the dispersion sponds to the reaction product of OH which comes inside the PEG-DA matrix owing to their higher ther- from H O and oxidized products of citrate produced dur- mal stability. The derivative thermo-gram (Fig.  7b) also 3+ ing the Au reduction [13, 22]. Figure 6b shows that the reveals the presence of three stage decomposition, cor- interactions of Au with PEG-DA were confirmed by pres - responding to the removal of water molecule, functional −1 ence of intense absorption bands at 1732 and 2882 cm moieties and decomposition of di-acrylate molecules. with their difference in intensity after functionalization process and dispersion in the PEG-DA matrix [23]. The Conclusions FT-IR spectra confirm the molecular structural changes In this study, a facile and rapid chemical method for between PEG-DA chains and functionalized Au nano- the uniform dispersion of plasmonic Au nanospheres spheres, ensuing by van der Walls interactions inside the into the PEG-DA matrix has been developed. Optical polymeric matrix. and spectroscopic analyses reveal the mono-dispersion Fig. 6 FT-IR spectroscopy of synthesized pristine Au/PEG-DA, lysine functionalized Au/PEG-DA, glutathione functionalized Au/PEG-DA, and thiogly- −1 −1 colic acid functionalized Au/PEG-DA in a 4000–800 cm range and b magnified 3300–1600 cm range near peaks from –C=O and C–H bonds of PEG-DA Ponnuvelu et al. Micro and Nano Syst Lett (2017) 5:21 Page 7 of 8 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 Received: 21 December 2016 Accepted: 20 April 2017 of plasmonic nanostructures into the polymer matrix, resulting in the formation of hybrid Au/PEG-DA hydro- gels. 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 stud - References 1. Yin PT, Shah S, Chhowalla M, Lee K-B (2015) Design, synthesis, and charac- ies, respectively. Chemical functionalization of the PEG- terization of graphene–nanoparticle hybrid materials for bio-applications. DA with functionalized Au nanospheres were observed Chem Rev 115:2483–2531 from the FT-IR studies by the existence of characteristic 2. 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Published: Apr 27, 2017

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