Suitable Polymeric Coatings to Avoid Localized Surface Plasmon Resonance Hybridization in Printed Patterns of Photothermally Responsive Gold Nanoinks

Abstract

When using gold nanoparticle (AuNP) inks for writing photothermal readable secure information, it is of utmost importance to obtain a sharp and stable shape of the localized surface plasmon resonance (LSPR) absorption bands in the prints. The T increase at a given irradiation wavelength (ΔTλ) is the retrieved information when printed patterns are interrogated with a laser source. As ΔTλ is proportional to the absorbance at the wavelength λ, any enlargement or change of the absorbance peak shape in a printed pattern would lead to wrong or unreliable reading. With the aim of preparing AuNP inks suitable for inkjet printing of patterns with stable and reliable photothermal reading, we prepared liquid solutions of spherical AuNP coated with a series of different polymers and with or without additional dispersant. The optical stability of the inks and of the printed patterns were checked by monitoring the shape changes of the sharp LSPR absorption band of AuNP in the visible (λmax 519 nm) along weeks of ageing. AuNP coated with neutral polyethylenglycol thiols (HS-PEG) of mw 2000–20000 showed a strong tendency to rapidly agglomerate in the dry prints. The close contact between agglomerated AuNP resulted in the loss of the pristine shape of the LSPR band, that flattened and enlarged with the further appearance of a second maximum in the Near IR, due to plasmon hybridization. The tendency to agglomerate was found directly proportional to the PEG mw. Addition of the ethylcellulose (EC) dispersant to inks resulted in an even stronger and faster tendency to LSPR peak shape deformation in the prints due to EC hydrophobicity, that induced AuNP segregation and promoted agglomeration. The introduction of a charge on the AuNP coating revelead to be the correct way to avoid agglomeration and obtain printed patterns with a sharp LSPR absorption band, stable with ageing. While the use of a simple PEG thiol with a terminal negative charge, HS-PEGCOO(−) (mw 3000), was not sufficient, overcoating with the positively charged polyallylamine hydrochloride (PAH) and further overcoating with the negatively charged polystyrene sulfonate (PSS) yielded AuNP@HS-PEGCOO(−)/PAH(+) and AuNP@HS-PEGCOO(−)/PAH(+)/PSS(−), both giving stable prints. With these inks we have shown that it is possible to write photothermally readable secure information. In particular, the generation of reliable three-wavelength photothemal barcodes has been demonstrated.

Harvesting Light To Produce Heat: Photothermal Nanoparticles for Technological Applications and Biomedical Devices

“…[205,206] Finally, the ability of the synthetic chemist to tailor the composition, shape dimensions and coating of NPs, and consequently their optical properties, photothermal response, and stability, allows complex, highly technological niche applications to be attained. These encompass the printing of photothermal anticounterfeit tags (e. g., for luxury goods or for encrypted writing), [199][200][201] the preparation of proteinaceous micropatterns with embedded photothermal NPs (suitable for high spatial resolution thermal stimulation of cells), [207] annealing of optical-fiber sensors bearing molecularly imprinted polymers with embedded plasmonic nanoparticles, [208,209] or photothermally driven tuning of the pore dimensions in polysulfone membranes for water filtration. [210]…”

“…High molecular weight thiolated poly(ethylene glycols) (PEG‐SH, M w 5000–20000) used as coatings are not successful, while an overlayer of PAH/PSS on a first coating layer of carboxylate‐terminated PEG‐SH allows to have sharp LSPR bands also in the prints, and to write photothermally readable secure information like a three‐wavelength photothermal barcode. [201] Also applications in biology or medicine have been envisaged for 2D printed photothermal NP. The CuS NP layers obtained on latex‐coated paper from Cu II acetate inks were proposed for microstructured patterns capable of thermal cell treatment, with no intrinsic toxicity, as human dermal fibroblasts can grow on top of the CuS films with no evidence of cytotoxicity.…”

PVA Films with Mixed Silver Nanoparticles and Gold Nanostars for Intrinsic and Photothermal Antibacterial Action

“…The LSPR band of AgNP in film-Ag remains instead well defined after 30 days with no significant changes. The behaviour of films containing GNS could be attributed to a residual mobility of the latter in the formed PVA matrix and to the tendency of PEG-coated Au nanoparticles to segregate in hybrid materials, a process that we have already observed for spherical Au nanoparticles [ 50 ]. Segregation and short GNS-GNS distances lead to plasmon hybridization and band flattening [ 51 , 52 , 53 ].…”

Nanoparticle-Imprinted Silica Gel for the Size-Selective Capture of Silver Ultrafine Nanoparticles from Water

“…We instead adopted the straightforward Turkevich approach [ 30 , 31 ] that gives reproducible access to citrate-coated Au-ufNP with d = 18 nm, Figure 2 A. These have an LSPR absorption with λ max 518 nm ( Figure 2 B) that shifts to 522 after coating with HS-PEG 2000 with the method described in [ 32 ].…”