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

Piersandro Pallavicini,Giuseppe Chirico,Chiara Milanese,Lorenzo De Vita,Angelo Taglietti,Francesca Merlin,Mykola Borzenkov

doi:10.3390/molecules25112499

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.

Highlights

Nanoparticles (NPs) capable of relaxing thermally when irradiated at the wavelength of their absorption bands add interesting photothermal properties to the many peculiarities of matter at the nanoscale
We recently proposed a radically different application of the photothermal effect of NIR-absorbing gold nanostars (GNS), i.e. Their use in a new approach in secure writing and anti-counterfeiting applications [11]
Are described in the this, the properties of are described in the Results and Discussion Section 2.4, that is dedicated to charged coatings, despite of the fact that HS2.4, that is dedicated to charged coatings, the fact that HS-PEGCOOH

Summary

Introduction

Nanoparticles (NPs) capable of relaxing thermally when irradiated at the wavelength of their absorption bands add interesting photothermal properties to the many peculiarities of matter at the nanoscale. NP of this kind are made of highly absorbing substances such as Prussian Blue [1], copper sulfide [2] or, more frequently, noble metals [3]. In the latter case, the absorption bands responsible of the photothermal effect are due to the well-known phenomenon of localized surface plasmon resonance (LSPR). Continuous irradiation of LSPR bands with laser or wide spectrum continuous sources leads to a largely prevalent thermal relaxation [5]. By decreasing the symmetry of Ag and Au nanoparticles from that of a sphere to those of elongated, branched or more complex shapes, such as nanorods, nanostars, nanoplates and nanoshells, the absorption maximum can be shifted from the visible to the Near-IR (NIR)

Methods

Results

Conclusion