Using Lock-In Thermography to Investigate Stimuli-Responsive Nanoparticles in Complex Environments

Abstract

The use of nanoparticles (NP) has been dramatically rising in the recent years and NPs can nowadays be found in various products ranging from food to composite materials or cosmetics [1,2]. Currently, the most frequently employed NP types are titania (TiO2), as a typical color additive, silica (SiO2), as anticoagulation agent, and silver (Ag) NPs, which are added to textiles, due to their antimicrobial properties. Because of their outstanding physical and mechanical properties carbon-containing nanomaterials, such as graphene and carbon nanotubes, have also experienced a surge in industrially relevant applications. About 30 % of the NPs are suspended in liquids, ranging from water to creams and lotions to car lubricants, followed by applications containing surface-bound (e.g., in textiles) NPs and finally nanocomposites (e.g., polymer-CNT composites). Together with biological and physiological fluids, these matrices render the detection and quantification of NPs fairly complex [3]. The broad range of chemical and biological compositions of these environments, including pH and ionic strength are often detrimental to the colloidal stability of NPs, potentially causing aggregation or even dissolution effects and therefore render NP analysis fairly challenging.Man-made, so-called 'engineered' NPs can be precisely tailored with respect to specific and desired properties. These materials are often stimuli-responsive meaning that they demonstrate the ability to generate heat upon a stimulus: alternating magnetic fields (AMF) for magnetic particles like superparamagnetic iron oxide NPs (SPIONs), and light in the UV, visible or NIR range for plasmonic NPs like gold spheres and other photothermal particles. Such a behavior

Quantification of nanoparticles' concentration inside polymer films using lock-in thermography

“…LIT makes use of the ability of some materials, such as plasmonic NPs or, in general, strongly optically absorbing components, to efficiently convert the absorbed energy of incident radiation into thermal energy. 41,43 The detection will depend on various factors, including the thermal properties of the material, the concentration of the absorbing material and the sensitivity of the IR-camera. LIT allows the detection of temperature variations on the surface of the sample in the millikelvin range.…”

“…39,40 Lock-in thermography (LIT) is an emerging non-destructive analytical technique for the investigation of stimuli-responsive nanomaterials. 41,42 It is conventionally employed in the analysis of solid lms, especially for applications in solar cells, and has the unique ability of providing a two-dimensional image of the sample emitting heat, thus allowing the assessment of the homogeneity of the samples. LIT makes use of the ability of some materials, such as plasmonic NPs or, in general, strongly optically absorbing components, to efficiently convert the absorbed energy of incident radiation into thermal energy.…”

“…This approach allows the signal-to-noise ratio to be signicantly improved and the spurious effects of background noise to be eliminated. 41,42,44 Other examples of thermographic techniques are vibrothermography (also known as ultrasonic thermography), 45 pulsed thermography (PT) 46 and pulsed phase thermography (PPT). 47 The main difference from LIT is the excitation process.…”

“…Indeed, LIT measurements can be carried out within minutes and do not require expensive setups, experienced users or complex data analysis. 41 The use of LIT has been recently demonstrated for the qualitative 48 and quantitative 49 determination of gold (Au) and silver (Ag) plasmonic NPs in suspensions. Moreover, LIT was successfully employed to detect and discriminate between aggregated and non-aggregated plasmonic NPs.…”