Spectral Reproducibility Research Articles

Ratiometric surface-enhanced Raman spectroscopy detection of 5-hydroxyindole-3-acetic acid based on Au@MIL-125@MIPs substrates

5-Hydroxyindole-3-acetic acid (5-HIAA) is a molecular marker that can be used in the early diagnosis of carcinoid tumors, and the development of sophisticated 5-HIAA assays is therefore of great importance. Surface-enhanced Raman spectroscopy (SERS) has been widely used for the rapid and sensitive detection of disease biomarkers. Insufficient specificity for tumor markers and poor spectral reproducibility are the bottlenecks in the practical use of SERS technology. In this study, based on MIL-125 surface-loaded gold nanoparticles (Au@MIL-125), a novel strategy was proposed to obtain Au@MIL-125@molecularly imprinted polymers (MIPs) as functional SERS substrates by wrapping a thin MIP shell around the Au@MIL-125 surface for selective separation followed by a 5-HIAA assay. The Raman peak intensity ratio (I865/I1078) was used to quantify 5-HIAA after a SERS spectral calibration with an embedded internal standard (i.e., 4-aminobenzenethiol) to improve the quantitative accuracy. The linear range was from 10−11 to 10−7 M, and the limit of detection (LOD) was 5.45 × 10−13 M. The method of integrating the MIPs with the metal MOF-based nanocomposites was shown to be useful in the analysis of real samples using SERS. The application of SERS for the selective and quantitative detection of analytes in real sample analysis, therefore, has great potential.

CFNet: Cross-modal data augmentation empowered fuzzy neural network for spectral fluctuation

Modern spectral analysis techniques are rapidly advancing, with Laser-induced breakdown spectroscopy (LIBS) gaining attention for its revolutionary potential in analytical chemistry. However, poor repeatability due to spectral fluctuation remains a common challenge. Improving LIBS repeatability involves improving instrument performance, standardizing sample handling, and refining data processing. While instrument performance and sample handling can be standardized, optimizing data processing is crucial for improving spectral reproducibility. This research addresses this issue through a 7-day experiment by proposing a cross-modal data augmentation empowered fuzzy neural network (CFNet). We first introduce a cross-modal data augmentation method that considers the spatial distribution of LIBS elemental lines. This method expands from a single spectrum modality to an image-spectrum dual modality, enhancing the ability to capture spectral fluctuation and thereby improving LIBS repeatability. We then introduce a cross-modal data augmentation empowered fuzzy neural network, which allows each spectrum to belong to multiple categories simultaneously, increasing adaptability to spectral fluctuation. Results show that both Accuracy and MacF exceed 91% across three tests, demonstrating the CFNet’s effectiveness in managing data fluctuation and serving as a reference for other spectral technologies. Integrating fuzzy logic into spectroscopy not only expands its applications but also improves the repeatability of spectral data. The cross-modal augmented data is available at https://github.com/aoao0206/CFNet.

The Impact of Heterogeneity of Aggregates Coated with Asphalt Mortar on Their FTIR Spectra and Spectral Reproducibility

Since FTIR is a sensitive micro-region measurement method, research on the impact of the heterogeneity of both aggregates and asphalt mortar is meaningful and comprehensive for accurate measurement with FTIR spectroscopy. In this paper, the impact of the heterogeneity of aggregates coated with asphalt mortar on their FTIR spectra and spectral reproducibility was creatively studied. The comparative analysis of the respective absorption peaks indicated that the characteristic absorption peaks of the aggregate coated with asphalt mortar were the superposition of the respective absorption peaks of its components. And research on the spectra of the coated aggregates obtained from the same batch of asphalt mixture fabricated at the same time showed that significantly different peak intensities could be affected by minor variations in their components due to the heterogeneity. Furthermore, statistical analysis suggested that the original spectral reproducibility of the coated aggregates was greatly affected by their heterogeneity, with a high coefficient of variation values. In conclusion, the heterogeneity of the coated aggregates could affect peak intensities and spectral reproducibility in micro-regions.

Reproducibility of the Optical Absorption Edge in Amorphous GeS2

Herein, poor reproducibility of optical absorption edges in GeS2 glasses and films is seen. Reported spectral positions of the absorption edge in melt‐quenched glasses spread over ≈0.2 eV at ħω ≈ 3 eV. In deposited films, the edge red‐shifts to ħω ≈ 2.5 eV showing wider variations of ≈1 eV. This work considers plausible reasons of such low, spectral reproducibility, with the aid of ab initio molecular orbital analyses of Ge–S clusters and known insights on optical gaps, electron‐spin‐resonance signals, and structural data. The variation in the glass is likely to be governed by several factors including compositional fluctuation, edge/corner‐shared configurations, wrong bonds, and intimate valence‐alternation pairs. The conspicuous red‐shift in the films seems to be affected also by neutral dangling bonds.

Uniform monolayer of branched silver nanoplates-assembled nanoparticles for sensitive and reproducible SERS detection of harmful molecules

Surface-enhanced Raman scattering (SERS) technique can offer the fingerprint information of trace substances, and thus has wide potential applications in many fields. There is still a requirement to develop new and effective methods to construct high-performance SERS substrates. Here, a facile electrodeposition method is presented for fabricating large-scale uniform films assembled by branched silver (Ag) nanoplates. The morphology and structure of such film are highly uniform over a large area, which can ensure good spectral uniformity and reproducibility. There are dense sub-10 nm gaps between neighboring Ag nanoplates and between neighboring branches, forming plenty of hot spots. Therefore, the Ag nanoparticle monolayer exhibits high SERS activity along with excellent spectral repeatability. It is found that the limits of detection (LODs) towards rhodamine 6G and 4-aminothiophenol can reach 0.387 pM and 49.0 pM, respectively, showing high sensitivity of the designed SERS substrate. The enhancement factor of the prepared roughened Ag nanoparticle monolayer is computed to be as large as 2.37 × 108. Then, the highly roughened nanoparticle monolayer is used to detect cationic dye and fungicide (methylene blue, MB) and a synthetic cytokinin (6-benzylaminopurine, 6-BAP), and the corresponding LODs are determined down to 1.1 nM and 28.8 pM, respectively. Such LODs are much lower than the tolerance for MB and 6-BAP residues established by some authoritative regulations. Sensitive monitoring of 6-BAP residue on vegetables and fruits can also be achieved using such SERS substrate, indicating its promising application in environment monitoring and food safety supervision.

Innovative Synthesis of Adhesive-Assisted and Recyclable Fe3O4@PD-Ag Photomagnetic Nanocomposites as SERS probes for Ultrasensitive Thiram Detection on Fruit Peels

Surface-enhanced Raman spectroscopy (SERS) with enormous advantages has emerged as a powerful tool for both highly sensitive structural detection of analytes and various imaging applications. A notable advancement in the realm of multifunctional SERS substrates is the integration of magnetic nanoparticles endowed with magnetic manipulation capabilities and noble metal nanoparticles exhibiting plasmonic resonance properties. This synergistic combination offers substantial practical benefits for trace analysis across diverse fields. In this paper, a new paradigm of Fe3O4@PEI-DTC-Ag (Fe3O4@PD-Ag) photomagnetic nanocomposites based on magnetic Fe3O4 nanoparticles (NPs), the noble metal silver quantum dots (Ag QDs) and the adhesive layer polyethyleneimine dithiocarbamate (PEI-DTC) for convenient recycling and ultrasensitive SERS detection was developed by engineering hydrothermal and seed-growth methods. The SERS performance of the Fe3O4@PD-Ag substrates with varying amounts of Ag was evaluated by detecting 4-aminothiophenol (4-ATP). Among the tested samples, Fe3O4@PD-Ag-4 with a silver solution volume of 200 ml exhibited the optimal performance, showcasing an enhancement factor (EF) of 4.418 × 105. Moreover, the relative standard deviations (RSDs) were calculated to be less than 11.5 %, demonstrating the SERS substrates exhibit good uniformity and spectral reproducibility. Of note, the SERS substrate has demonstrated successful trace detection of thiram molecules, achieving a limit of detection (LOD) of up to 10-9 M. The intensity of SERS signals shows a linear correlation with the logarithm of thiram concentration, ranging from 10-4 to 10-9 M, thus providing compelling evidence for quantitative detection. Importantly, the Fe3O4@PD-Ag-4 SERS substrate also showcases outstanding performance for detecting thiram in real food, underscoring its significant potential for application in food safety monitoring.

Laser constructed vacancy-rich TiO2-x/Ti microfiber via enhanced interfacial charge transfer for operando extraction-SERS sensing

Semiconductor-molecule surface-enhanced Raman scattering (SERS), especially the stronger interfacial charge transfer process (ICTP), represents a frontier in the field of SERS with spectral reproducibility and unparalleled selectivity. Herein, through a laser microfabrication method in situ, the free-standing, super hydrophilic and vacancy-rich TiO2-x/Ti is successfully synthesized. Using blue TiOx/Ti (B-TiOx/Ti) as pre-concentrated substrate, a nanomolar-level limit of detection of 12 nmol/L at 1385 cm–1, is confirmed using crystal violet (CV) bacteriostat as a model under 532 nm excitation. Furthermore, the results demonstrate that the SERS enhancement mechanism is via the moderate adulteration of oxygen vacancy, which leads to a narrow value of band gap and increases the ICTP of substrate to molecules. Using a hand-held extractor assembled with B-TiOx/Ti microfiber, the operando analysis of mixtures distributed information excited in different parts of Asian carp is facilely achieved. This work guides the controlled synthesis of vacancy-rich TiO2-x/Ti nanostructure and its application in ultrasensitive extraction-SERS detection. It also provides the direction for the rapid and operando transmission of biological information with temporal and spatial concentration distribution in human tissues by highly sensitized materials.

Structure-regulated enhanced Raman scattering on a semiconductor to study temperature-influenced enantioselective identification.

Surface-enhanced Raman scattering (SERS) spectroscopy is an effective technique that can reveal molecular structure and molecular interaction details. Semiconductor-based SERS platforms exhibit multifaceted tunability and unique selectivity to target molecules as well as high spectral reproducibility. However, the detection sensitivity of semiconductors is impeded by inferior SERS enhancement. Herein, a surface and interference co-enhanced Raman scattering (SICERS) platform based on corrugated TiO2 nanotube arrays (c-TiO2 NTs) was developed, and the coupling of structural regulation and photo-induced charge transfer (PICT) effectively optimized the SERS performance of the semiconductor substrate. Due to the regularly oscillating optical properties of the c-TiO2 NTs, well-defined interference patterns were generated and the local electric field was significantly increased, which greatly promoted both the electromagnetic mechanism and PICT processes. The c-TiO2 NTs were subsequently applied as a highly sensitive SICERS substrate to investigate the mechanism of temperature influence on enantioselective identification. This identification process is related to the existence of temperature-sensitive hydrogen bonds and π-π interaction. This work demonstrates a simply prepared, low-cost, and sensitive SERS substrate that enables better investigation into molecular identification.

High energy facet-dominated TiO2−X facet heterojunction with excellent carrier utilization for ultrasensitive SERS sensing and efficient degradation of antibiotic residues

Realizing efficient utilization of photogenerated carriers is crucial to boosting the photophysical and photochemical performances of semiconductors. Herein, (101) and (001) facets co-exposed anatase TiO2−X facet heterojunction with abundant oxygen vacancies was developed as a new surface-enhanced Raman scattering (SERS) substrate and also as an excellent photocatalyst for synchronous detection and degradation of antibiotic residues in actual water system. Due to the efficient carrier separation induced by facet heterojunction and the efficient charge migration derived from exposure of more high energy (001) facets, the TiO2−X facet heterojunction exhibits an ultrahigh SERS enhancement factor of 1.07 × 107 for non-resonant Raman probe, with excellent spectral reproducibility and stability. More meaningfully, the developed substrate can realize an ultrasensitive detection for different kinds of antibiotic residues in real river water, even a comprehensive identification for multicomponent residues. Moreover, TiO2−X facet heterojunction can also execute an efficient photocatalytic degradation for residue pollutants in actual water system. And, the developed multi-function substrate possesses an excellent recyclability not only in SERS detection but also in photocatalysis.

V2O5 nanoparticle films as a platform for plasmon-free surface-enhanced Raman spectroscopy

Metal-oxide nanomaterials represent a promising and inexpensive alternative to plasmonic metallic nanostructures currently used for surface-enhanced Raman scattering (SERS) spectroscopy. They profit from a chemical enhancing mechanism, i.e., charge transfer between the adsorbed molecule and the substrate. In this study, SERS-active nanostructured films based on vanadium pentoxide (V2O5) are produced by the combination of gas-phase synthesis of nanoparticles followed by their thermal annealing, i.e., using a fully solvent- and linker-free procedure. The SERS performance of V2O5 nanoparticle films was found to be strongly linked with porosity and the appropriate crystalline structure adjusted by annealing. A Raman spectroscopy and X-ray diffraction revealed that V2O5 crystalline nanoparticles are formed only after the annealing at temperatures equal to or higher than 300 °C. Such produced V2O5 nanoparticle films allow for sensitive and reproducible detection of organic dyes (methylene blue, rhodamine 6G and crystal violet), while their SERS enhancement (analytical enhancement factor close to 103) is significantly higher as compared with magnetron sputtered and annealed V2O5 thin films. Our study represents an important step in designing novel plasmon-free SERS-active platforms with the aim of improving their stability and spectral reproducibility compared to other non-plasmonic and plasmonic ones.

G-C3N4/TiO2-X heterojunction with high-efficiency carrier separation and multiple charge transfer paths for ultrasensitive SERS sensing

The combination of interface engineering and defect engineering is a promising strategy for developing new semiconducting surface-enhanced Raman scattering (SERS) substrate. Herein, an organic/inorganic hybrid g-C3N4/TiO2-X heterojunction with synchronous generation of strong interface effect and abundant surface oxygen vacancy (OV) defect was prepared by a simple sol-hydrothermal procedure with a help of urea. Due to the improved substrate-to-molecule charge transfer (CT) from joint contribution of high-efficiency carrier separation induced by strong interface coupling effect and multiple CT paths derived from abundant surface OV, g-C3N4/TiO2-X substrate exhibits greatly enhanced SERS effect for non-resonant 4-mercaptobenzoic acid (4-MBA) probe. The enhancement factor of g-C3N4/TiO2-X substrate for 4-MBA is as high as 5.57×106, and the substrate exhibits ultra-high stability and excellent spectral reproducibility. More meaningfully, the developed g-C3N4/TiO2-X heterojunction can be used to execute an ultrasensitive detection for antibiotic residues in real water system, even comprehensive evaluation of multi-component residues.

Bleach Cleaning of Commercially Available Gold Nanopillar Arrays for Surface-Enhanced Raman Spectroscopy (SERS).

Surface-enhanced Raman spectroscopy (SERS) is a highly sensitive technique that can assist in trace analysis for biomedical, diagnostic, and environmental applications. However, a major limitation of SERS is surface contamination of the substrates used, which can complicate the spectral reproducibility, limits of detection, and detection of unknown analytes. This is especially prevalent with commercially available substrates as shipping under a controlled and clean environment is difficult. Here we report a method using dilute bleach solutions to remove surface contamination from commercially available substrates consisting of gold-coated nanopillar arrays that maintains functionality. The results show that this method can be used to remove background signals associated with typical surface contamination in commercially available substrates as well as remove thiolated self-assembled monolayers (SAMs). Results indicate the bleach oxidizes the surface contaminants, which can then be easily washed away. Although the metallic surface also becomes oxidized in this process, the surface can be reduced without loss of SERS activity. The SERS intensity of SAMs improved following bleach treatment across all concentrations studied.

A Surface-Enhanced Raman Spectroscopic Sensor Pen

Surface-enhanced Raman spectroscopy (SERS) is widely used as a detection method in scientific research fields. However, the method for creating SERS substrates often requires expensive equipment and involves a complex process. Additionally, preserving and effectively utilizing SERS substrates in the long term poses a challenging problem. In order to address these issues, we propose a new method for creating SERS substrates on various types of paper using a combination of a ballpoint pen and 3D printing. This method ensures a high enhancement factor and maximizes the utilization of the substrate. We achieved an enhancement factor of up to 8.2 × 108 for detecting R6G molecules, with a relative standard deviation of 11.13% for the Raman peak at 612 cm−1 of R6G, demonstrating excellent SERS sensitivity and spectral reproducibility. Furthermore, we successfully detected thiram at a concentration as low as 10−8, which is lower than both the Chinese national standard and European standard.

Sensitive and reproducible on-chip SERS detection by side-polished fiber probes integrated with microfluidic chips

The combination of surface-enhanced Raman scattering (SERS) and microfluidic technology offers a promising solution to improve the reproducibility and reliability of SERS detection. Optical fiber SERS probes, as an important class of SERS substrates, provide a large SERS interaction area and flexible integration with microfluidic chips. In this article, a side-polished multimode fiber (SPMF) SERS probe is prepared by depositing Au nanorods on a planar polished fiber surface and integrating it into a microfluidic channel to form a glass-based microfluidic-SERS chip. The impact of SPMF residual thickness on SERS performance is thoroughly investigated, and the optimized residual thickness of 62 μm is determined by considering the evanescent wave intensity, transmission loss, and experimental feasibility. The optimized microfluidic-SERS chip shows low detection limits (as low as 10-8 M for MG) and good spectral reproducibility (RSD less than 10%) due to the large SERS interaction area provided by the SPMF SERS probe and the stable liquid flow in the microfluidic channel. The chip is furtherly used to detect pesticide and antibiotic residues in tap water, with the detection limits of 10-9 M for thiram and 10-6 M for levofloxacin. This work provides an effective way to realize highly sensitive and reproducible SERS detection, and may find important applications in environmental science, food safety, and biomedicine.

Double-enhanced LIBS system with N2 atmosphere and cylindrical cavity confinement for quantitative analysis of Sr element in soil

To effectively improve the quality of laser-induced plasma spectra and the detection accuracy of laser-induced breakdown spectroscopy (LIBS) simultaneously for Sr element in soil, the dual mechanism of N2 atmosphere combined with cylindrical cavity confinement for laser-induced Sr plasma emission enhancement was proposed and investigated in this paper, and a multi-spectral fusion internal standard analysis model was established. Optimum enhancement effect can be achieved under the conditions of 2 mm diameter and 6 mm height of the confinement cavity in N2 atmosphere, where the enhancement factor was about 3.25, the signal-to-noise ratio reached 710.28, and the LIBS spectrum quality was the best; the relative standard deviation value (2.64%) was the smallest, and the LIBS signal reproducibility was the best. For the samples in this study, the limit of detection of the sample Sr elements under the dual enhancement mechanism was 34.60 mg kg−1, which was 40.43% lower than the limit of detection (LOD) without enhancement mechanism (58.08 mg kg−1), and the R 2 of the multispectral fusion internal standard model was 1.23% higher and the relative error was 3.41% lower than that of the internal standard method. The results showed that the dual enhancement mechanism combining N2 atmosphere and cylindrical cavity confinement improved the spectral quality, signal reproducibility, and detection sensitivity.

Fabrication of Vertically Aligned ZnO Nanorods Modified with Dense Silver Nanoparticles as Effective SERS Substrates

Surface-enhanced Raman scattering (SERS) spectroscopy has attracted increasing attention due to its high spectral reproducibility and unique selectivity to target molecules. Here, a facile approach is proposed to prepare Ag nanoparticles modified ZnO nanorod arrays (Ag/ZnO NR arrays). Ag nanoparticles were densely decorated on the surface of ZnO nanorods through silver mirror reaction and subsequent seed-assisted electrodeposition. The prepared Ag/ZnO NR arrays can be used as a sensitive, uniform, and repeatable SERS substrate for the rapid detection of organic dye molecules and biomolecules with concentrations higher than the corresponding limits of detection (LODs). The LODs for rhodamine 6G (R6G), 4-aminothiophenol (PATP) and adenine are calculated to be 1.0 × 10−13 M, 1.6 × 10−12 M and 3 × 10−11 M, respectively. The enhancement factor (EF) of the SERS substrate is estimated to be as high as ~2.7 × 108 when detecting 10−10 M R6G. Particularly, the as-synthesized substrate exhibits high selectivity to multiple components. In addition, the fabricated Ag/ZnO NR arrays can be recycled due to their superior self-cleaning ability and can realize photocatalytic degradation of R6G in water within 1 h driven by UV light, showing that the three-dimensional recyclable SERS substrates have wide applications in environmental pollution monitoring and biomedical analysis.

Rapid detection of paraquat residues in fruit samples using mercaptoacetic acid functionalized Au@AgNR SERS substrate

An optimum SERS performance is mainly dependent on the development of innovative substrates that show high spectral reproducibility and stability. The current study involves the synthesis of core–shell nanorods (Au@AgNRs) functionalized with mercaptoacetic acid (MAA) to increase the stability and sensitivity of the substrate for cost-effective and sensitive assessment of paraquat (PQ) residues in fruit samples. The morphological analysis of the substrate using TEM and STEM-EDS images indicated the uniformity in size and structure of the prepared nanoparticles. Further the modified substrate, MAA/Au@AgNRs showed excellent stability and sensitivity in comparison with its unmodified counterpart. For the evaluation of PQ residues in pear and apple fruit samples the modified substrate exhibited R2 values of 0.9954 and 0.9914 with a low limit of detections of 0.012 and 0.024 ppm, and good percent recoveries of 80–110 % and 81–104 %, respectively. Thus, the study proved the effectiveness of the proposed MAA/Au@AgNRs substrate for assessing residues of herbicide in complex foods.

Responsive Spiral Photonic Structures for Visible Vapor Sensing, Pattern Transformation and Encryption

AbstractTwo photon polymerisation using direct laser writing is a burgeoning field of research, with recent focus being placed on bringing added value to microstructures, by incorporating soft, responsive polymers. Moving to the micron‐scale can have a profound impact on such stimuli‐responsive materials, whose speed of actuation can be increased many‐fold compared to their mm‐scale counterparts. Here, the fabrication of submicron 2D photonic structures, based on a vapor‐responsive photoresist with a refractive index <1.55, in the visible wavelength range. The fabricated concentric spiral arrays are evaluated for their feasibility as vapor sensors by testing spectral and structural color reproducibility and reversibility under water, ethanol, isopropanol, and acetone vapors. This approach allows for the realization of predictable uniform color displays that can be modulated upon stimuli response. The transmitted colour in the dry and hydrated states can be accurately modelled. This knowledge is used to design and demonstrate structures for cloaking and image transformation. Such capability can be used for encryption and anti‐counterfeiting applications.

Unusual Spectrally Reproducible and High Q-Factor Random Lasing in Polycrystalline Tin Perovskite Films.

An unusual spectrally reproducible near-IR random lasing (RL) with no fluctuation of lasing peak wavelength is disclosed in polycrystalline films of formamidinium tin triiodide perovskite, which have been chemically stabilized against Sn2+ to Sn4+ oxidation. Remarkably, a quality Q-factor as high as ≈104 with an amplified spontaneous emission (ASE) threshold as low as 2µJcm-2 (both at 20K) are achieved. The observed spectral reproducibility is unprecedented for semiconductor thin film RL systems and cannot be explained by the strong spatial localization of lasing modes. Instead, it is suggested that the spectral stability is a result of such an unique property of Sn-based perovskites as a large inhomogeneous broadening of the emitting centers, which is a consequence of an intrinsic structural inhomogeneity of the material. Due to this, lasing can occur simultaneously in modes that are spatially strongly overlapped, as long as the spectral separation between the modes is larger than the homogeneous linewidth of the emitting centers. The discovered mechanism of RL spectral stability in semiconductor materials, possessing inhomogeneous broadening, opens up prospects for their practical use as cheap sources of narrow laser lines.

Self-assembled gold nanoparticles on the serpentine networks of Calf Thymus-DNA Langmuir-Blodgett films as efficient SERS sensing platform: Fabrication and its application in thiram detection

Fabrication of efficient SERS active substrates from the Langmuir-Blodgett (LB) films of Calf Thymus-DNA (CT-DNA) molecule after incubation into gold nanocolloid for 24 h has been presented. Relaxation kinetics of CT-DNA molecules at the air-water interface has been explicitly studied. The experimental observations from UV–Vis electronic absorption spectra together with the FESEM and AFM images of CT-DNA molecule organized in LB film collectively suggest the presence of H-type aggregated domains due to plane-to-plane self-association of the CT-DNA molecules on the surface of the LB film. XPS spectrum of the as prepared substrate has been unveiled to identify its chemical compositions while their morphological features are explored from the FESEM and AFM images. The SERS efficacy of the substrate has been tested with the 4-Mercaptopyridine (4-MPy) molecule at trace concentration. SERS enhancement factors ranging from 106–107 orders of magnitude have been evaluated for the characteristic bands of the 4-MPy molecule. The substrate not only exhibits appreciable spectral reproducibility, but it also confirms its stability and uniformity. The as prepared SERS active substrate has also been used for thiram detection at ultrasensitive concentration.