Synergistic Enhancement of Photothermal Conversion and Mechanical Properties in PVA Hydrogels via Co-Doping of Polydopamine and Gold Nanoparticles
ChaoqunJiang1,2
WenyanZhang1,2✉Emailwiseyanyan@jit.edu.cn
YujieZhang3
HuiwenYuan3
1College of Material EngineeringJinling Institute of technologyNanjingChina
2College of Architectural EngineeringJinling Institute of technologyNanjingChina
3Nanjing Institute of Product Quality InspectionNanjingChina
Chaoqun Jiang1,2, Wenyan Zhang1,*z,2, Yujie Zhang3, Huiwen Yuan3
1 College of Material Engineering, Jinling Institute of technology, Nanjing, China
2 College of Architectural Engineering, Jinling Institute of technology, Nanjing, China
3 Nanjing Institute of Product Quality Inspection, Nanjing, China
z,2 Corresponding author: Wenyan Zhang Email: wiseyanyan@jit.edu.cn
Abstract
Photothermally responsive hydrogels with excellent mechanical properties are highly desirable for applications in biomedical engineering, soft actuators, and wearable devices. This work offers a simple and effective strategy for constructing photothermally responsive hydrogels with enhanced mechanical behavior by incorporating polydopamine (PDA) and citrate-stabilized gold nanoparticles (Au NPs) via a freeze-thaw process. Polydopamine functioned as a broadband photothermal agent, while Au NPs provided localized surface plasmon resonance (LSPR) effects. In addition, both polydopamine and Au NPs acted as physical crosslinking points and stress transfer centers within the hydrogel network. The incorporation of Au NPs also facilitated charge carrier migration, further contributing to improved photothermal conversion. As a result, compared with pure PVA hydrogels, the PVA@PDA@Au hydrogels exhibited significantly enhanced photothermal conversion efficiency after three freeze-thaw cycles, owing to the synergistic effect between polyPDAmine, Au NPs and PVA network. Moreover, the mechanical strength was remarkably improved, with the maximum load, tensile strength, and yield stress increasing by 4.6-fold, 4.2-fold, and 8-fold, respectively.
Keywords:
Photothermal hydrogel
Polyvinyl alcohol (PVA)
Polydopamine
Au NPs
Freeze-thaw process
Mechanical reinforcement
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1. Introduction
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In recent years, hydrogels has gained research interest for photothermal conversion, which has potential in the fields of solar interfacial evaporation, biomedical application, and wound treatment [,]. Among hydrogels, polyvinyl alcohol (PVA) hydrogels have attracted extensive attention in biomedical engineering, soft electronics, and smart materials due to their biocompatibility, environmental friendliness, tunable tissue-like mechanical properties, excellent transparency, and responsive swelling behavior [,,,].However, conventional PVA hydrogels intrinsically suffer from several limitations for photothermal conversion applications. Firstly, PVA hydrogels has low mechanical strength and toughness insufficiency[,]. Besides, owing to their non-conjugated polymer backbone and lack of intrinsic chromophores, pristine PVA hydrogels exhibit low absorption in the near infrared region (NIR) window, resulting in insufficient photothermal conversion efficiency for NIR irradiation[,]. Furthermore, under continuous NIR laser exposure, single-component PVA hydrogels are prone to thermal relaxation, network degradation, and photothermal induced volume transitions, which compromise their mechanical integrity and limit operational reliability for long-term use [,]. These drawbacks severely restrict the practical applicability of PVA-based hydrogels as efficient, stable photothermal materials in clinical and wearable optoelectronic systems.
To address these challenges, this study provides a facile, scalable strategy for fabricating multifunctional, biocompatible hydrogels with synergistic photothermal and mechanical properties. Polydopamine and citrate-stabilized gold nanoparticles (Au NPs) are synergistically incorporates into the PVA hydrogel to form a composite PVA-based hydrogel. As a catecholamine-based biopolymer, polydopamine not only offers strong adhesion and reactive functional groups for anchoring Au NPs and PVA chains, but also possesses intrinsic broadband light absorption and photothermal properties [,,]. Meanwhile, citrate-stabilized Au NPs exhibit intense localized surface plasmon resonance (LSPR) effects for enhancing light harvesting and photothermal conversion via strong photothermal coupling [,,]. Compare with PVA hydrogels, the designed composite hydrogel demonstrated over 32% improvement in photothermal response and the mechanical performance increased by 8 times.
2. Experiment
2.1 Sample Preparation and Matrix Formulation
Citrate-stabilized Au NPs were synthesized via the reduction of 0.01 mM HAuCl4 with 1% (w/v) sodium citrate under boiling for 10 min. Sodium citrate served as both a reducing and stabilizing agent, ensuring uniform particle size and colloidal stability of Au NPs. High-molecular-weight polyvinyl alcohol (PVA, ≥ 99% hydrolyzed) was dissolved in a mixed solvent of glycerol, dimethyl sulfoxide (DMSO), and deionized water (volume ratio 1:2:10). The mixture was stirred at 95°C until fully dissolved, then cooled to room temperature. Polydopamine solution (5 mg/mL, pH 8.5) and citrate-stabilized gold NPs (Au NPs) were added into the mixture in sequence. The final solution was cast into silicone molds and subjected to three freeze-thaw cycles (-20°C/25°C, 24 h per cycle) to form physically crosslinked hydrogels.
2.2 Characterization
During photothermal cycling performance test, the samples were irradiated with a 275 W infrared lamp. Surface temperature was recorded in real time using a K-type thermocouple (diameter 0.1 mm, accuracy ± 0.1°C). Each cycle involved 30 min continuous irradiation followed by 5 min measurement. To evaluate mechanical performance, Quasi-static uniaxial tensile tests were conducted on a 506A universal testing machine (500 kg/5000 N load cell) following ASTM D638-V standards. Specimens were loaded at 20 mm/min until fracture, with force-displacement data recorded at 100 Hz. A field emission scanning electron microscope (FESEM, Zeiss Sigma 300) was employed to investigate the morphology and particle size of the prepared samples. Infrared reflection spectra were obtained using a Nexus 870 spectrometer, while UV-vis-NIR absorption spectra were measured with a Shimadzu UV-2600i spectrophotometer.
3. Results and discussion
3.1 Morphology and elemental characterization of hydrogel
The cross-sectional morphology of the PVA-based hydrogel are shown in Fig. 1. As seen in Fig. 1(a), the pure PVA hydrogel exhibits a porous structure with irregularly distributed cavities, which may be attributed to the phase separation and water crystallization during the freeze-thaw process. After incorporating polydopamine (Fig. 1(b)), the porous structure disappears, and the hydrogel displays a denser, more uniform morphology. This suggests that polydopamine interacts with the PVA network, enhancing the physical crosslinking and suppressing pore formation. In Fig. 1(c), the PVA@PDA@Au hydrogel shows an smoother cross-section compared to PVA@PDA, indicating the introduction of citrate-stabilized Au NPs further improves the structural uniformity of the hydrogel network. This may result from additional interactions between the Au NPs, polydopamine, and PVA chains, which strengthen the hydrogel matrix and inhibit microstructural defects.
Fig. 1
Cross section morphology of PVA-based hydrogel (a) PVA (b) PVA@PDA and (c) PVA@PDA@Au after three freeze-thaw process
a.
(d) SEM image of PVA@PDA@Au hydrogel; (e-h) C, O, N, Au elemental mapping of PVA@PDA@Au hydrogel matrix shown in (d)
(i)
UV-vis-NIR absorbance spectrum of citrate-stabilized Au NPs
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The elemental distribution of the PVA@PDA@Au hydrogel was analyzed by energy-dispersive X-ray spectroscopy (EDS) mapping recorded from the sample shown in Fig. 2(d). Figure 2(e-h) respectively present the corresponding elemental mappings of carbon (C), oxygen (O), gold (Au), and nitrogen (N) of Fig. 2(d). The C element originates from PVA and polydopamine, O element comes form PVA, N is derived from polydopamine, and Au is contributed by the incorporated Au NPs. It could be observed that the Au and N elements are uniformly distributed within the PVA hydrogel matrix. In addition, the Fig. 2(f) shows the UV-vis-NIR absorbance spectrum of the citrate-stabilized Au NPs dispersed in distilled water. The distinct absorption peak at approximately 520 nm is characteristic of nano-scaled Au particles which intrinsically possess localized surface plasmon resonance (LSPR).3.3 Photothermal response of hydrogel
The incorporation of polydopamine and Au NPs improved the photothermal performance of PVA-based hydrogel, as confirmed by the photothermal test. Figure 2 (a1,b1,c1) shows the temperature elevation of the prepared hydrogels illuminated by NIR irradiation, recorded for PVA, PVA@Au, PVA@PDA and PVA@PDA@Au hydrogel after one, two and three freeze-thaw cycles. Figure 2 (a2,b2,c2) presents the temperature summit during the photothermal test for the hydrogels prepared via one, two and three freeze-thaw cycles. PVA@PDA@Au hydrogel exhibited enhanced photothermal behavior than PVA, PVA@Au, and PVA@PDA. Compared with PVA, a temperature elevation of 3o, 10o and 12o was achieved on PVA@PDA@Au through freeze-thaw of different cycles, indicating polydopamine and Au NPs functioned as photothermal agents within the PVA matrix.
Fig. 3
Photothermal response of PVA-based hydrogels and Temperature summit during the tests
(a1) Photothermal response and (a2) Temperature summit of hydrogels in one freeze-thaw cycle
(b1) Photothermal response and (b2) Temperature summit of hydrogels in two freeze-thaw cycle
(c1) Photothermal response and (c2) Temperature summit of hydrogels in three freeze-thaw cycle
Moreover, as indicated in Fig. 3, the photothermal effect of PVA@PDA@Au hydrogels became more pronounced with increasing of freeze-thaw cycles. FTIR spectra were recorded to investigate this performance. As shown in Fig. 4, new absorption bands appeared at 1013 and 954 cm− 1 after successive freeze-thaw cycles. These bands indicates increased hydrogen bonding and microcrystalline ordering following repeated freeze-thaw processing, which promote polymer chain aggregation and intermolecular interactions. The denser network structure not only limits heat dissipation but also increases the light path length within the hydrogel, resulting in higher photothermal conversion efficiency. In addition, multiple freeze-thaw cycles promote stronger interfacial interactions between PVA chains, polydopamine, and Au NPs, which reduces thermal resistance at the filler-matrix interfaces and enhances thermal conductivity and heat transfer efficiency within the composite network. Thereby, photothermal response of the PVA@PDA@Au hydrogel network was improved with freeze-thaw cycles.
Fig. 4
FTIR spectra of PVA@PDA@Au hydrogels of one and three freeze-thaw cycle
Fig. 5
(a) UV-vis-NIR absorbance of PVA@PDA@Au, PVA@PDA, and PVA hydrogels of three freeze-thaw cycle; (b) I-V plots of PVA@PDA@Au and PVA@PDA hydrogels of three freeze-thaw cycle
Compared with PVA, the superior performance of PVA@PDA@Au could be is attributed to the synergistic photothermal conversion capability of polydopamine and Au NPs. As shown in Fig. 5(a), PVA@PDA@Au hydrogels exhibit enhanced light absorption compared to PVA@PDA and pure PVA hydrogel in the visible (300–700 nm) and near-infrared (700–800 nm) regions. The increased absorption is attributed to the strong broadband absorption of polydopamine, together with the LSPR effect of the incorporated Au NPs. Moreover, as shown in Fig. 5(b), the conductivity of PVA@PDA@Au is approximately 3 times of that of PVA@PDA, indicating the migration of charge carriers are facilitated in the matrix modified by Au NPs.
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Scheme 1
Illustration of photothermal effect in (a) PVA, (b) PVA@PDA, and (c) PVA@PDA@Au
As illustrated in Scheme 1(a,b), polydopamine uniformly distributed within the PVA hydrogel acts as an efficient photothermal agent, owing to its broadband absorption and the photothermal conversion capability of catechol and indole groups, which dissipate absorbed photons into heat via rapid non-radiative relaxation pathways. Meanwhile, as shown in Scheme 1(c), the LSPR effect of Au NPs complements the light absorption of polydopamine and generates additional photothermal heating. Moreover, the incorporation of Au NPs enhances the electrical conductivity of the hydrogel network, facilitating rapid thermal diffusion and reducing localized heat accumulation under irradiation. In addition, both polydopamine and Au NPs serve as multifunctional physical crosslinking points, synergistically reinforcing the hydrogel network and promoting efficient stress and heat transfer pathways within the matrix. As a result, the PVA@PDA@Au hydrogel exhibits markedly superior photothermal and mechanical performance compared to PVA@PDA and pure PVA hydrogels, as demonstrated in Scheme 1.
3.2 Mechanical performance of hydrogel
Figure 6 and Table 1 present the mechanical properties of PVA-based hydrogels after successive freeze-thaw cycles. The results confirm that the mechanical performance of PVA hydrogels was significantly enhanced by the incorporation of polydopamine and Au nanoparticles. Both PVA@PDA and PVA@Au hydrogels exhibited increased maximum load (Fmax), tensile strength, and yield stress compared to pure PVA hydrogels. This improvement is attributed to the role of polydopamine and citrate-stabilized Au nanoparticles as physical crosslinking points and stress transfer centers within the PVA network. Through hydrogen bonding interactions, these components effectively restricted polymer chain mobility and promoted more uniform stress distribution under applied load, as illustrated in Scheme 2(b,c).
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Fig. 6
Mechanical performance of PVA-based hydrogels after (a) one, (b) two, and (c) three freeze-thaw cycles
Cycle | Sample | Fmax (N) | Yield Stress (MPa) | Tensile Strength (MPa) |
|---|---|---|---|---|
1 | PVA | 0.64 | 0.01 | 0.06 |
PVA@PDA | 1.34 | 0.03 | 0.13 | |
PVA@Au | 1.65 | 0.04 | 0.16 | |
PVA@PDA@Au | 2.94 | 0.08 | 0.25 | |
2 | PVA | 0.35 | 0.01 | 0.03 |
PVA@PDA | 2.58 | 0.04 | 0.22 | |
PVA@Au | 2.06 | 0.04 | 0.17 | |
PVA@PDA@Au | 4.08 | 0.08 | 0.34 | |
3 | PVA | 1.25 | 0.05 | 0.10 |
PVA@PDA | 1.15 | 0.02 | 0.10 | |
PVA@Au | 2.03 | 0.04 | 0.17 | |
PVA@PDA@Au | 4.08 | 0.08 | 0.34 |
PVA@PDA@Au, containing both polydopamine and citrate-stabilized Au nanoparticles, exhibited superior mechanical performance compared to the other hydrogels. After the first freeze-thaw cycle, their Fmax, yield stress, and tensile strength increased by approximately 4.6-fold, 8-fold, and 4.2-fold, respectively, relative to pure PVA hydrogels. This remarkable enhancement is attributed to the strong interfacial interactions among PVA chains, polydopamine, and Au nanoparticles, which act synergistically as physical crosslinking points and stress transfer centers within the network, as illustrated in Scheme 1(d).
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Scheme 2
Structure illustration of PVA-based hydrogels(a) pristine PVA, (b) PVA@PDA,
(c) PVA@Au, and (d) PVA@PDA@Au
More importantly, the Fmax, tensile strength, and yield stress of all hydrogels progressively increased with the number of freeze-thaw cycles, as shown in Fig. 6 and Table 1. As evidenced by the FTIR spectra (Fig. 4), repeated freeze-thaw processing promoted the formation of additional microcrystalline domains and hydrogen-bonded junction zones within the PVA matrix, which served as supplementary physical crosslinking points alongside polydopamine and Au nanoparticles. These structural reinforcements further enhanced the integrity of the hydrogel network and facilitated more efficient stress transfer under mechanical loading.
4 Conclusion
In summary, multifunctional PVA-based hydrogels with enhanced photothermal conversion efficiency and mechanical properties were successfully fabricated through the incorporation of polydopamine and citrate-stabilized gold nanoparticles via a freeze–thaw process. The synergistic effect between polydopamine and Au nanoparticles, combined with progressive network densification through successive freeze-thaw cycles, significantly improved the photothermal performance and mechanical strength of the hydrogels. Specifically, the PVA@PDA@Au hydrogels exhibited a 12 oC increase in photothermal conversion after three freeze-thaw cycles, along with remarkable enhancements in maximum load, tensile strength, and yield stress by 4.6-fold, 4.2-fold, and 8-fold, respectively, compared to pure PVA hydrogels. This work provides a simple and effective strategy for designing high-performance photothermally responsive hydrogels with excellent mechanical behavior, offering potential for applications in wearable electronics, soft actuators, and biomedical devices.
Corresponding author: Wenyan Zhang
Funding Declaration
The authors appreciate the Natural Science Fund of Jiangsu Province (BK20221167), Qinlang Project of Jiangsu Province, and Nanjing Optometric New Materials and Application Technology Innovation Team.
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Acknowledgement
Dr. Wenyan Zhang appreciate spiritual support from her parents in heaven.
Data and code availability
The data that supports the findings of this study are available within the article.
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Author Contribution
Author contributionsThe corresponding author is responsible for ensuring that the descriptions are accurate and agreed by all authors.Chaoqun Jiang: Methodology, Visualization, Investigation, Formal analysis, Writing Original DraftWenyan Zhang: Conceptualization, Supervision, Methodology, Writing-original draft, Editing, Investigation, Project administration, Funding acquisitionYujie Zhang: Methodology, Visualization, Investigation, Formal analysisHuiwen Yuan: Formal analysis, Resources, Investigation
Wenyan Zhang
Conceptualization, Supervision, Methodology, Writing-original draft, Editing, Investigation, Project administration, Funding acquisition
Chaoqun Jiang
Methodology, Visualization, Investigation, Formal analysis, Writing Original Draft
Yujie Zhang
Methodology, Visualization, Investigation, Formal analysis
Huiwen Yuan
Formal analysis, Resources, Investigation
Conflict of interest statement
On behalf of all authors, the corresponding author states that there is no conflict of interest.
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