Thermal Lens Microscope Research Articles

Recent Progress and Applications of Thermal Lens Spectrometry and Photothermal Beam Deflection Techniques in Environmental Sensing.

This paper presents recent development and applications of thermal lens microscopy (TLM) and beam deflection spectrometry (BDS) for the analysis of water samples and sea ice. Coupling of TLM detection to a microfluidic system for flow injection analysis (μFIA) enables the detection of microcystin-LR in waters with a four samples/min throughput (in triplicate injections) and provides an LOD of 0.08 µg/L which is 12-times lower than the MCL for microcystin-LR in water. μFIA-TLM was also applied for the determination of total Fe and Fe(II) in 3 µL samples of synthetic cloudwater. The LODs were found to be 100 nM for Fe(II) and 70 nM for total Fe. The application of µFIA-TLM for the determination of ammonium in water resulted in an LOD of 2.3 µM for injection of a 5 µL sample and TLM detection in a 100 µm deep microfluidic channel. For the determination of iron species in sea ice, the BDS was coupled to a diffusive gradient in the thin film technique (DGT). The 2D distribution of Fe(II) and total Fe on DGT gels provided by the BDS (LOD of 50 nM) reflected the distribution of Fe species in sea ice put in contact with DGT gels.

Photothermal spectroscopy and micro/nanofluidics

Photothermal spectroscopy, a remarkable detection method that can analyze microscale objects in a noninvasive and nondestructive manner, has been successfully coupled with micro/nanofluidic devices. Specifically, methods that employ a thermal lens microscope (TLM), including a photothermal optical phase shift and photothermal optical diffraction, are a powerful tool for the sensitive detection of nonfluorescent or nonlabeled molecules in micro/nanofluidic channels. This review focuses on the family of TLMs in terms of their historical development. Their recent applications, ranging from separation, particle, biomedical, energy, and environmental analyses, are summarized, and future perspectives in nanoscale liquid science, system integration, and biological studies, such as single-cell analyses, are also discussed.

A comparison between thermal lens and conventional optical spectroscopy for monitoring of a photocatalytic process

We compare the thermal lens (TLS) and the optical transmission (OT) spectroscopy techniques to monitor the kinetic of a photocatalytic reaction. For this, an OT measurement facility was added to a TLS set-up. The TLS was implemented in a microspatial configuration named thermal lens microscopy (TLM). Methylene blue (MB) in Water solutions were used as test samples within a concentration range in which both techniques show good sensibility. Within this range, the limit of detection obtained by TLM was about one order of magnitude lower than that achieved by OT. The methylene blue concentration evolution with photocatalytic reaction time was measured with both techniques, showing a good agreement between their results. A ZnO thin film deposited on a glass substrate by the spray pyrolysis technique was used as catalyst, and the reaction was induced by UV-violet light.

Online electrophoretic nanoanalysis using miniaturized gel electrophoresis and thermal lens microscopy detection

Online thermal lens microscopy (TLM) coupled with gel electrophoresis (GE) can represent a powerful tool for separating and detecting a wide range of biomaterials. Unlike slab gel electrophoresis (SGE), the proposed method does not require prolonged procedure between separation and detection. In this work, we developed an online monitoring GE system to separate and detect nanosized materials. The design is based on a homemade and cost-effective miniaturized GE chip (MGEC) integrated with real-time TLM detection through microcontroller-based digitization board platform. To validate the feasibility and practicability of the proposed approach, we evaluated its separation capability via employing synthesized Fe3O4-Au core-shell nanoparticles (NPs) which served remarkably for the proof-of-concept. The optimum conditions for the separation process were achieved through optimization of the excitation power as 30 mW, detection position at 24 mm, the concentration of agarose gel 0.5 % w/v, and 37.5 V/cm as the effective electric field strength. The findings showed that two populations of Fe3O4–Au, core-shell, and uncapped Fe3O4 NPs, were effectively separated in less than eleven minutes, demonstrating rapid assessment of the nanomaterial production quality. Moreover, other characterization techniques such as HRTEM and EDX were employed to confirm the presence of the two dissimilar kinds of NPs separated using MGEC-TLM. The sensitivity of the method was demonstrated by determining the limit of detection (23 pM) for 10 nm AuNPs. It is envisaged that our presented system enables rapid, economical, low volume of reagents consumption and high potential analysis for quality test in various bioanalytical and nanotechnological applications.

Design optimization of microfluidic-based solvent extraction systems for radionuclides detection

The development of reliable and fast automated methodologies to detect and identify radionuclides during the decommissioning of nuclear power plants is of paramount importance. In this regard, process flowsheeting and computational simulations are useful tools to aid the design and testing of these advanced detection technologies. We implement an optimization based design procedure for the design of continuous analysis systems based on microfluidic solvent extraction and on-line measurement to detect radionuclides in nuclear waste. The optimization of such detection systems is treated as a design under uncertainty problem. The systems are based on thermal lens microscopy as the detection instrument. We demonstrate our approach on a flowsheet for the detection of trivalent lanthanides in organic and aqueous solutions. We highlight the importance of using computer-aided optimization based procedures to design microsystems comprising several chemical operations and their coupling with the detection step. It constitutes a proof of concept and a first step towards robust optimization based modelling approaches for the design of microfluidic lab-on-a-chip platforms for the detection of radionuclides in nuclear waste.

Review of ultrasensitive readout for micro-/nanofluidic devices by thermal lens microscopy

Thermal lens microscopy (TLM) utilizes the effects, such as changes in the refractive index, caused by heat generated in the sample for the detection of nonfluorescent analytes with at least a hundred-fold enhancement in sensitivity compared with optical absorbance spectrometry. Micro-/nanofluidic devices can provide specificity and sample manipulation capabilities. The integration of these two technologies exposes the potential to attain the holy grail of continuous, real-time, label-free, specific, and ultrasensitive detection, which find applications in environmental monitoring, quality control of chemical manufacturing, single-cell analysis, and biomedicines. Here, we summarize the recent advances in the instrument development and innovative applications, and suggest future directions of research of TLM.

Development of a Hyphenated Method Called DLLME/TLM for Trace Analysis of Cd

Dispersive Liquid-Liquid Microextraction / Thermal Lens Microscopy (DLLME/TLM) was developed as a new combination method for preconcentration and determination of Cd. Thermal Lens Microscopy is suitable for the determination of analyte after DLLME because of the low volume of the remained phase after DLLME and increasing of the enhancement factor for the nonpolar organic solvents. Some effective parameters on the micro extraction, complex formation and combination were selected and optimized. Under optimum conditions, the calibration graphs were linear in the range of 0.05-20 µg/L with the detection limit of 0.008 µg/L. The Relative Standard Deviation (RSD) for 1 and 10 µg/L of cadmium was 3.1 and 2.3, respectively. The enhancement factor of 1200 was obtained from a sample volume of 10.0 mL. DLLME/TLM method was applied to the analysis of real samples. The accuracy of the method was proved by using standard reference materials and micro spectrophotometry.

Thermal Lens Microscope and Microchip Chemistry

Abstract Thermal lens spectrometry was realized under an optical microscope by controlling the chromatic aberration of the objective lens system. This thermal lens microscope (TLM) is ultrasensitive to non-fluorescent molecules and detection at even the yoctomole level in a liquid was confirmed. The TLM is widely applicable, almost the same as spectrophotometry, but its specificity for the analyte species is similarly poor. During development of the TLM equipment, a microchannel fabricated on a glass substrate was used as a sample vessel, which then became a microchemical chip. The generalized microchemical chip design method, micro unit operation (MUO) and continuous flow chemical processing (CFCP) was proposed and a variety of complex chemical processes for analyses, syntheses, and biomedical experiments could be integrated onto the microchemical chip. Therefore, the development of microchemical chips realized significant expansion of the field of microfluidics. Microchemical chips are also suitable devices for separation chemistry, and combination of the TLM and microchemical chips could compensate for the poor selectivity of TLM. These innovative methods will not only impact analytical chemistry but also microfluidics and most certainly other research fields such as biology and medicine. This account of The Chemical Society of Japan Award for Creative Work 2006 introduces these achievements.

Application of thermal lens microscopy (TLM) for measurement of Cr(VI) traces in wastewater

In this work, we demonstrate for the first time that Thermal Lens Microscopy technique (TLM) can be applied to monitor the dynamics of a photocatalytic process in-situ. The photocatalytic reduction of hexavalent chromium -Cr(VI)- in aqueous solution using CdS and irradiated with visible light is monitored by TLM. Since the values of Cr(VI) concentration obtained after the photocatalytic process were close to those imposed by the international regulations for drinking water, the use of TLM allowed its measurement with a better reliability than with UV spectroscopy, usually used in this kind of analysis.

Photothermal CH vibrational overtone detection by two‐color laser absorption

AbstractIn this experiment, the thermal lens technique is used with a modified arrangement of three lasers to induce a two‐color absorption. Two‐pump lasers, a variable wavelength orange dye laser (588–617 nm), a single line blue laser (488 nm), and a single line probe yellow laser (568 nm) are employed. A comparison is made between the magnitude of the thermal lens signal obtained with a one pump laser versus two pump lasers. The absorbing molecules are benzene and naphthalene in liquid n‐Hexane. The CH vibrational overtone spectra are obtained at room temperature for several concentrations. The molecules are excited to a high vibrational state (Δυ = 6) with the first laser and to an electronic level with a second laser (two‐color absorption). Using two pump lasers, the limit of detection of the molecule is several orders of magnitude more sensitive than using one pump laser. A nonlinear behavior of the integrated signal versus concentration is shown for the two‐color laser process. Linear behavior is shown for the one pump laser experiment. A model of signal amplification for a nonlinear absorption is presented to explain the results. The separation and identification of CH overtone bands in molecules and the sensitivity of the technique is emphasized to convey the potential use of CH overtone spectroscopy for imaging in thermal lens microscopy.

Focal-space intensity distribution of an incoherent light source beam behind a focusing lens

Based on ray-tracing technique and vectorial diffraction theory, focal-space intensity distribution of an incoherent light source (ILS) beam after a focusing lens is theoretically described. We investigated the influence of different parameters (the source size, wavelength, focusing lens type, etc.) on the focal-space intensity distribution. For a point source beam, a hyperbolic lens provides 50-fold better axial intensity concentration than a spherical lens of the same focal length (10 mm). Under a chromatic hyperbolic lens, broadening of the spectral bandwidth induces an elongation of the axial intensity distribution, which however is not obvious for spherical lens focusing since the spectral broadening-induced elongation is partly overlapped with that induced by the spherical aberration. Increasing the source size will broaden the lateral intensity distribution, which first looks like a Gaussian and then evolves into a hat top. The axial and/or lateral extension of focal-space intensity would be beneficial to applications where volume heating rather than point heating is required. For example, in an ILS-excited thermal lens microscopy, extension of the intensity distribution brings a quadratic increase of photothermal signal of an ensemble sample but a linear increase of the signal for a point-like target.

In-situ monitoring by thermal lens microscopy of a photocatalytic reduction process of hexavalent chromium

In this work, we describe the application of a micro-spatial thermal lens spectroscopy setup (thermal lens microscope, TLM) with coaxial counter-propagating pump and probe laser beams and an integrated passive optical Fabry-Perot to quantify the Cr-VI concentration in water during a photocatalytic reaction in-situ. A series of test samples was analyzed using the 1,5 diphenil carbazide colorimetric method. A calibration curve was obtained by plotting of the TLM signal as a function of the concentration of Cr(VI) in a range between 0 and 10 μg/L (1 μg/L = 1 ppb, part per billion), with a detection limit of 53 ng/L (1 ng/L = 1 ppt, part per trillion). A solution of 10 μg/L Cr(VI) in distillated water was placed into a cell in contact with an iron-incorporated titanium dioxide film, which was previously grown onto a 1 mm thick glass microscope slide by the sol-gel dip-coating technique. The TLM signal was registered as a function of the photocatalysis time measured from the beginning of the process, radiating the film with UV-violet light. The Cr(VI) concentration was determined with the calibration curve and after the first 50 minutes a reduction of 95 % of Cr(VI) was observed, being the chemical reaction kinetic described by a potential time decreasing function.

Molecular Counting of Captopril by a Microfluidic Chip-Thermal Lens Microscopy System

Background: The photo thermal lens microscopy is a technique based on measurement of the temperature rise which is generated in an illuminated matrix because of non-radiative relaxation of absorbed energy from a laser. It is an ideal system for low detection volume measurements in a micro channel. Hence, in this study, a combined microfluidic chip-photo thermal lens microscopy (MFCPTLM) system was assembled for highly sensitive determination of captopril. Captopril is an angiotensin-converting enzyme (ACE) inhibitor used to treat hypertension, congestive heart failure and kidney problem caused by diabetes. Captopril had no absorption in visible range but due to its thiol group, it could interact with Gold nanoparticles (GNPs). So, GNPs act as its labeling agent and the semi-selective measurement of captopril was achieved. Methods: A 3.35 nM GNPs and captopril solution by the same flow rate were introduced to the microchannels by microsyringe pumps. The mixture was detected with an assembled PTLM system. The applicability of optimized method for determination of captopril in human Serum and pharmaceutical sample was investigated as well. Results: The thiol group of captopril bonded to the surface of GNPs, created the core-shell form and reduced the surface plasmon resonance of GNPs; as a consequence, the PTLM signal of GNPs decreased following the increase in captopril concentration. The difference between the PTLM signals of sample and blank was calculated as a ΔPTLM signal. The detection volume of the PTLM was calculated to be 2.68 fL. At the optimum condition this optical view comprised of 5.4 GNPs, surrounded by 65-1049 captopril molecules. Conclusion: In this study, we proposed a sensitive method for the determination of captopril, based on photo thermal lensing of GNPs in a glass microfluidic chip. The S-type form of microchannel and the micro space scale of that enhanced the surface interaction of captopril molecules and GNPs and resulted in more effective reaction. In addition to the fast interaction time and low detection volume, the main features of this method are simplicity, good analytical performance, and low reagent consumption in comparison with batch procedures. The combination of this system with a highly sensitive method such as PTLM reduced the difficulty of sampling which makes it more attractive. The results show the capability of the method for sensitive measurement of captopril in human serum sample and pharmaceutical tablets. Keywords: Microfluidic chip (MFC), photo thermal lens microscopy (PTLM), captopril, gold nanoparticles (GNPs), surface plasmon resonance (SPR), and molecular scale measurement.

A flexible thermal lens microscope for highly sensitive detection in microfluidic chips

A flexible thermal lens microscope (TLM), which combines the advantages of both conventional thermal lens spectrometry and TLM, is reported in this paper. The flexibility lies in the fact that the TLM can be configured in diffraction-limited (DL) or non-DL excitation mode: in small micro spaces (<50 µm), the setup can work in DL mode (as in previous TLMs) and shows a limit of detection (LOD) of 2 × 10‒5 cm‒1; in large micro spaces (50–1000 µm),the setup working in non-DL mode was demonstrated to be much more advantageous. Compared with the case in DL mode, the TLM in non-DL mode shows up to an eight times lower LOD under 100–1000-fold less intense excitation, and better resistivity to flow-induced signal reduction and fluctuation to beam misalignment and background scattering, making it particularly suitable for detecting photolabile samples in highly flowing scattering mediums. For photostable samples, excitation laser powers of watt scale (in contrast to 100 mW in DL mode) could be employed to further decrease the LOD. This flexible TLM can therefore be applied for highly sensitive on-chip analysis of a broad range of analytes in different mediums.

High sensitivity thermal lens microscopy: Cr-VI trace detection in water

In this work, a low detection limit for hexavalent chromium in water of parts per trillions (21ng/L) was achieved using a micro-spatial thermal lens spectroscopy setup with coaxial counter-propagating pump and probe laser beams and an integrated passive optical Fabry-Perot resonator, aided with a well-established diphenyl carbazide colorimetric method. Cr-VI concentrations in the range of μg/L, i.e. well-below the toxicity thresholds in humans and animals (26 and 190mg/L respectively) and below those delimited by international regulations for drink water (~0.05–0.5mg/L), have been obtained by measurements in bottled and tap water samples. The developed thermal lens microscope is also capable to detect Cr-VI directly in potassium dichromate solutions using pump beam wavelengths within the very low optical absorption region in the visible part of the spectrum, i.e., without the use of any colorimetric method.

Laser induced thermal lens microscopy for highly sensitive determination of captopril.

In this work, a combined flow injection-photo thermal lens microscopy (FI-PTLM) system was used for highly sensitive determination of captopril as an angiotensin-converting enzyme inhibitor. Captopril has no absorption in the visible range, but due to its thiol group could interact with gold nanoparticles (GNPs). GNPs, because of their surface plasmon resonance (SPR), have absorption in the visible range, but their interaction with a low concentration of captopril shows no effective change in UV-Vis spectrophotometry because their aggregation is slight. On the contrary, at the same condition, the PTLM with a visible light source enables sensitive measurement of this compound. The thiol group of captopril binds to the surface of GNPs and decreases the SPR. At the optimum condition in the focal volume of 2.68 fL (f=10-15), the obtained range of linearity was 50-800 nM. The developed method was successfully applied for the determination of captopril in human serum and pharmaceutical samples.

Femtoliter Scale Detection of an Antithyroid Drug Using Gold Nanoparticles in a Microfluidic Chip

Sensitive detection of methimazole (MMI) at the femtoliter (fL) scale has been achieved based on photo thermal lensing of gold nanoparticles (GNPs). GNPs were used because their surface plasmon resonance (SPR) have absorption in visible range. A combined glass microfluidic chip-photo thermal lens microscopy (MFC-PTLM) system with appropriate laser source was assembled for detection of MMI with GNPs. The thiol group of MMI bound to the surface of GNPs and created the core-shell form and reduced the surface plasmon resonance of GNPs as a consequence the PTLM signal of GNPs decreased following the increase of MMI concentration. The results showed that the changes of PTLM signal in the focal volume of 2.68 fL were proportional to the concentration of MMI over the range of 75-900 × 10-9 mol L-1. The obtained detection limit of this method was 46.5 × 10-9 mol L-1 and the relative standard deviation for ten repeated measurements of 550 × 10-9 mol L-1 MMI solutions was 3.47%. The developed method was successfully applied for determination of MMI in human serum and pharmaceutical samples.

Determination of Trace Cr(VI) with Diphenylcarbazide by μFIA-Thermal Lens Microscopy.

The optimum reaction parameters for the interaction of hexavalent chromium [Cr(VI)] with diphenylcarbazide in microfluidic chips (μFIA) with thermal-lens microscopic detection were selected. The characteristic feature of the applied flow scheme is the injection of the reagent into the stream containing the test metal, which enables in-field and real-time monitoring of Cr(VI) simply by flowing the sample continuously through the microchip. The limit of detection of Cr(VI) under the selected conditions (signal generating wavelength, 514.5 nm; excitation power, 100 mW; detection position, 10 cm downstream from the mixing zone of the microchip; flow rate 10 μL min-1; injection volume, 1.4 μL) is 15 ng mL-1 (2.9 × 10-7 mol L-1). The linear range is 40 ng mL-1 - 10 μg mL-1 with a relative standard deviation no higher than 10% in the concentration range 0.1-1 μg mL-1. The online monitoring by this scheme provides the possibility of up to 360 analyses per hour.

Thermal lens spectrometry in electromigration methods of analysis

We consider the basic principles, instruments, problems, and examples of application of thermal lens spectroscopy and microscopy, highly sensitive force methods of molecular absorption spectroscopy, in various embodiments of electromigration methods of analysis, particularly, capillary electrophoresis and microfluidic chip electrophoresis. Examples illustrating the sensitivity and selectivity of the combined methods are presented.

Thermal lens microscope sensitivity enhancement using a passive Fabry–Perot-type optical cavity

We developed a thermal lens microscope equipped with a passive optical cavity, which provides an optical feedback for the multiple pass of the probe laser beam to enhance sensitivity. Considering the maximum absorption peak for Fe(II) at 532 nm wavelength, we have achieved a 6.6-fold decrease in the limit of detection (LOD) to a level of 0.077 μg · l−1 without a cavity. The possibilities to use thermal lens detection combined with an optical resonator was proposed and a drastic thermal lens signal enhancement was achieved using very low excitation power. The corresponding LOD for Fe(II) was further decreased to the level of 0.006 μg · l−1 which represents an 85-fold decrease of the LOD value. This setup is a promising device, which can be applied as a sensitive tool for detecting chemical traces in small volumes of solutions.