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Highlights • Sensitivity of thermal lens microscopy doesn't necessarily show a 1/κ dependence. • Ratio of thermal diffusion length to excitation radius is key to κ-dependent signal. • Detection of ensemble sample shows a different κ-dependence from that of nanoparticle. • Volumetric heat capacity is also a key parameter impacting the photothermal signal. Abstract Thermal conductivity (κ) is a key factor influencing the sensitivity of photothermal techniques. In thermal lens spectroscopy (TLS), photothermal signal was previously regarded as being proportional to 1/ κ. Here, I show that the 1/ κ dependence of photothermal signal is only valid on condition that the thermal diffusion length (L D) exceeds quintuple the excitation beam radius (a e) for an ensemble sample or twice the probe beam radius (r ps) for a nanoparticle. When L D is smaller than 0.5 a e for ensemble sample (or r ps /6 for nanoparticle), the signal reaches a plateau (or starts to decrease), totally deviating from the 1/ κ dependence. It was experimentally demonstrated that at high modulation frequencies, the signal enhancement induced by organic solvent can be twice smaller than that predicted previously. This work illuminates the impact of κ on TLS, and provides a guidance on selecting optimal TLS parameters for sensitive nonfluorescent target detection. [ABSTRACT FROM AUTHOR]

The thermal lens technique is proposed for the determination of total hemoglobin in the form of reversible hemichrome. The conditions were optimized (concentration of sodium dodecyl sulfate, 2 mM) to attain the maximum sensitivity with the use of polyethylene glycols as signal enhancers. For polyethylene glycols with molecular weights 1500-35000 Da in a concentration range of 5-15% w/w (5-25 mM), the influence on thermal lens signal enhancement was estimated. It is shown that the use of 5% w/w polyethylene glycol 2000 provides the maximum increase in the thermal lens enhancement factor (by 40%) in comparison with unmodified aqueous solutions. The detection limit of iron(II) tris(1,10-phenanthrolinate) as a model system is 60 nM. Under these conditions, the thermal lens detection limit of hemichrome is 10 nM, which shows a 15-fold enhancement compared to spectrophotometry. Modification of the medium with polyethylene glycols decreases the limit of detection of hemichrome determination by 15% in comparison with unmodified aqueous solutions due to better reproducibility for the range of concentrations from 0.02 to 0.9 µM. [ABSTRACT FROM AUTHOR]

In this article, the historical development of thermal lens spectrometry (TLS) is briefly reviewed as an introduction. In continuation, the emphasis is on the recent progresses of TLS for measurements in ensembled sample cells and in microfluidic flow injection systems. Novel theories, instrumentation and their applications for high sample throughput for environmental, chemical and biomedical analysis, as well as thermal characterization and imaging, particularly in microspace, are presented. Discussion is given on the limitations of present TLS systems that open new horizons for future progress of this technique, which has already found place among routine techniques for chemical analysis. In the final section, proposals for the future development of TLS towards advanced applications in new research fields are presented. [ABSTRACT FROM AUTHOR]

A combined microfluidic flow injection analysis-thermal lens microscopy ( $$\upmu $$ FIA-TLM) system was applied for determination of neutrophil gelatinase-associated lipocalin (NGAL)-a biomarker of acute kidney injury. NGAL was determined following a commercial ELISA assay and transfer of the resulting solution into the $$\upmu $$ FIA-TLM system with a 100 $$\upmu $$ m deep microchannel. At an excitation power of 100 mW, the $$\upmu $$ FIA-TLM provided about seven times lower limits of detection (1.5 pg $${\cdot }\mathrm{mL}^{-1})$$ as compared to a conventional ELISA test, and a sample throughput of six samples per minute, which compares favorably with sample throughput of the microtiter plate reader, which reads 96 wells in about 30 min. Comparison of results for NGAL in plasma samples from healthy individuals and for NGAL dynamics in patients undergoing coronary angiography measured with transmission mode spectrometry on a microtiter plate reader and with $$\upmu $$ FIA-TLM showed good agreement. In addition to improved LOD, the high sensitivity of $$\upmu $$ FIA-TLM offers possibilities of a further reduction of the total reaction time of the NGAL ELISA test by sacrificing some of the sensitivity while reducing the duration of individual incubation steps. [ABSTRACT FROM AUTHOR]

The optical configuration of a thermal lens microscope (TLM) was optimized for detection in a microfluidic chip with respect to the flow velocity, and the pump and probe beam parameters (beam waists, offsets, and mode mismatching degree). It was found that an appropriate pump-probe beam offset for a certain flow velocity would provide not only a higher sensitivity but also a better response linearity of TLM over three orders of magnitude of sample concentration. Diffraction-limited pump beam excitation is advantageous for space-resolved measurement, while a larger pump beam with 10 times lower power density is favorable for higher sensitivity at given experimental conditions. As an application, TLM was used to study the diffusion of azobenzene in a microfluidic chip. Diffusion profiles at different distances from the mixing point were recorded by scanning the TL signal along the cross section of the microchannel. By fitting the diffusion profiles to a theoretical model of mass transfer in a microchannel, diffusion coefficients of azobenzene in octane and methanol were determined to be $$5\times 10^{-10}\,\hbox {m}^{2}{\cdot }\,\hbox {s}^{-1}$$ and $$6\times 10^{-10}\,\hbox {m}^{2}{\cdot }\,\hbox {s}^{-1}$$ , respectively. [ABSTRACT FROM AUTHOR]

Thermal lens spectrometry (TLS) under excitation of a divergent pump beam is discussed in both conventional and microscopic TLS instruments. A refined thermal lens (TL) model was proposed for calculating the TL signal of a 'finite TL element.' Experiments as well as comparison with numerical simulations demonstrated that the effective sample length for a certain pump beam profile was about six times the confocal distance of the pump beam for laser-excited case and 1.5 mm for incoherent light source-excited case. For laser-excited conventional TLS instrument or thermal lens microscope (TLM), an empirical formula of the optimum pump beam waist radius for maximum detection sensitivity was obtained at a given sample length. At larger pump beam waist radius of 7 μm, the TL signal was found 2.5 times lower compared to the diffraction limit; however, the resulting two orders of magnitude lower power density in the sample could be quite desirable for the detection of photolabile analytes. By investigating the influence of a finite TL element on the TL signal, we found that an optimal distance between the probe beam waist and the sample was needed to assure the maximum detection sensitivity and good response linearity. Under the optimal detection scheme, limit of detection of the laser-excited TLM at 4 mW power was evaluated to be 8.6 × 10 M for 100-μm-thick ferroin solution, corresponding to an absorbance of 9.5 × 10 absorbance units. [ABSTRACT FROM AUTHOR]