Gold nanomakura: nanoarchitectonics and their photothermal response in association with carrageenan hydrogels

Abstract

Photothermal conversion of light into heat energy is an intrinsic optical property of metal nanoparticles when irradiated using near-infrared radiation. However, the impact of size and shape on the photothermal behaviour of gold nanomakura particles possessing optical absorption within 600-700 nm as well as on incorporation in hydrogels is not well reported. In this study, nanomakura-shaped anisotropic gold nanoparticles (AuNMs) were synthesized via a surfactant-assisted seed-mediated protocol. Quaternary cationic surfactants having variable carbon tail length (n = 16, 14, 12) were used as capping for tuning the plasmon peak of gold nanomakura within a 600-700 nm wavelength. The aspect ratio as well as anisotropy of synthesized gold nanomakura can influence photothermal response upon near-infrared irradiation. The role of carbon tail length was evident via absorption peaks obtained from longitudinal surface plasmon resonance analysis at 670, 650, and 630 nm in CTAB-AuNM, MTAB-AuNM, and DTAB-AuNM, respectively. Furthermore, the impact of morphology and surrounding milieu of the synthesized nanomakuras on photothermal conversion is investigated owing to their retention of plasmonic stability. Interestingly, we found that photothermal conversion was exclusively assigned to morphological features (i.e., nanoparticles of higher aspect ratio showed higher temperature change and vice versa irrespective of the surfactant used). To enable biofunctionality and stability, we used kappa-carrageenan- (k-CG) based hydrogels for incorporating the nanomakuras and further assessed their photothermal response. Nanomakura particles in association with k-CG were also able to show photothermal conversion, depicting their ability to interact with light without hindrance. The CTAB-AuNM, MTAB-AuNM, and DTAB-AuNM after incorporation into hydrogel beads attained up to ≈17.2, ≈17.2, and ≈15.7 °C, respectively. On the other hand, gold nanorods after incorporation into k-CG did not yield much photothermal response as compared to that of AuNMs. The results showed a promising platform to utilize nanomakura particles along with kappa-carrageenan hydrogels for enabling usage on nanophotonic, photothermal, and bio-imaging applications.

Keywords: anisotropy; hydrogel; kappa-carrageenan; metal nanoparticles; nanoarchitectonics; nanomakura; photothermal properties; surfactants.

Figure 1

(a) Seed-mediated synthesis of CTAB-AuNM, MTAB-AuNM, and DTAB-AuNM, a two-step reduction method showing gold (Au) seed preparation and formation of AuNMs in growth medium. (b) Absorption spectra of different surfactant-capped Au seeds over a wavelength range of 300–900 nm. (c) Absorption spectra of CTAB-AuNM, MTAB-AuNM, and DTAB-AuNM, respectively, over a wavelength range 300–900 nm.

Figure 2

Stability of AuNMs (CTAB-AuNM, MTAB-AuNM, and DTAB AuNM) in different media at the ratio (NPs:media) 3:1 where (a), (b), and (c) represent the stability of AuNMs in ethanol (70%), NaCl (0.9%), and 1X phosphate buffer saline (PBS).

Figure 3

(a) k-CG hydrogel beads (HB) without AuNMs and with incorporated AuNMs; (b), (c), and (d) show stable absorption spectra of CTAB-AuNM, MTAB-AuNM, and DTAB-AuNM in k-CG hydrogels at different time intervals.

Figure 4

TEM micrographs along with histograms showing morphology and size (length) of (a) CTAB-AuNM, (b) MTAB-AuNM, and (c) DTAB-AuNM, respectively; (d), (e), and (f) showed AFM images of CTAB-AuNM, MTAB-AuNM, and DTAB-AuNM, respectively, along with their 3D topography.

Figure 5

(a) FTIR spectra of CTAB-AuNM, MTAB-AuNM, and DTAB-AuNM, respectively. (b) XRD diffractogram showing diffraction peaks of CTAB-AuNM, MTAB-AuNM, and DTAB-AuNM, respectively.

Figure 6

Growth mechanism of gold nanomakura particles (AuNM). (a) AuNM surrounded by surfactant micelles; (b) growth facet of AuNM; and (c) stepwise growth mechanism during AuNM formation.

Figure 7

(a) Seed-mediated synthesis of CTAB-capped AuNR. (b) Seedless synthesis of DTAB-capped AuNR. (c) Absorption spectra of CTAB-AuNR and DTAB-AuNR, respectively, over a range of 300–900 nm.

Figure 8

Anisotropic gold nanoparticles showing photothermal response of: (a) CTAB-AuNM, MTAB-AuNM, and DTAB-AuNM in colloidal state; (b) CTAB-AuNR and DTAB-AuNR in colloidal state; (c) CTAB-AuNM, MTAB-AuNM, and DTAB-AuNM in solid state (powder form); (d) CTAB-AuNM, MTAB-AuNM, and DTAB-AuNM after incorporation within k-CG hydrogel beads (HB) (powder form).

Figure 9

(a) Temporal variation in the corresponding temperatures on photothermal interaction with a visible broadband light source in DTAB- and CTAB-capped AuNR in powdered form. (b,c) Corresponding thermal images for temperature rise in AuNR capped with DTAB and CTAB, respectively, at time = 0 s (initial) and 300 s (after irradiation).

Figure 10

Thermal images showing photothermal response in (a) DTAB-AuNR and (b) CTAB-AuNR after incorporation into k-CG hydrogels at time = 0 s (initial), and 300 s (after irradiation).

Figure 11

Thermal images showing photothermal response in (a) CTAB-AuNM, (b) MTAB-AuNM, and (c) DTAB-AuNM after incorporation into k-CG hydrogels at time = 0 s (initial) and 300 s (after irradiation).