Luminescent carbon dots versus quantum dots and gold nanoclusters as sensors.

Ultra-small nanoparticles, including quantum dots, gold nanoclusters (AuNCs) and carbon dots (CDs), have emerged as a promising class of fluorescent material because of their molecular-like properties and widespread applications in sensing and imaging. However, the fluorescence properties of ultra-small gold nanoparticles (i.e., AuNCs) and CDs are more complicated and well distinguished from conventional quantum dots or organic dye molecules. At this frontier, we highlight recent developments in the fundamental understanding of the fluorescence emission mechanism of these ultra-small nanoparticles. Moreover, this review carefully analyses the underlying principles of ultra-small nanoparticle sensors. We expect that this information on ultra-small nanoparticles will fuel research aimed at achieving precise control over their fluorescence properties and the broadening of their applications.

Fluorescent Nitrogen-doped Carbon Dots-based Turn-off Sensor for Bilirubin.

Bilirubin (BR), a heme protein produced from breakdown of haemoglobin is present in aged red blood cells; whose abnormal concentration is associated with diseases like hyperbilirubinemia, coronary disease, iron deficiency, and so on. Herein, we have synthesized a selective, sensitive, and low-cost sensing platform using fluorescent nitrogen doped carbon dots (NCDs), prepared from precursors; citric acid and urea via a simple microwave-assisted method. The emission at 444 nm on excitation with 360 nm was well quenched in presence of BR suggesting a direct turn-off detection for BR. Characterization of developed probe was done by UV-Visible absorption studies, photoluminescence studies, SEM, TEM, ATR-FTIR, XPS, and DLS analysis. BR was detected with a Limit of Detection (LoD) and Limit of Quantification (LoQ) of 0.32 µM and 1.08 µM respectively. NCDs exhibited excellent selectivity and sensitivity towards BR in the presence of co-existing biomolecules and ions. Practical feasibility was checked by paper-strip-based sensing of BR and spiked real human samples were used for conducting real sample analysis.

Terbium Phenanthroline Complex as a Luminescent Probe for the Detection of Anthrax Biomarker: Dipicolinic acid.

Dipicolinic acid (DPA) is a prominent biomarker for Anthrax disease. Bacillus anthracis bacterial endospores is composed of DPA as the significant component, which on over inhalation can cause severe health issues. Such contagious and life-threatening pathogens can be employed as bioweapons or biothreat agents for spreading bioterrorism which is a major risk for national security and public health concerns. Hence, effective detection or a surveillance system is essential for preventing the growth of bioterrorism events. Herein, we have developed a Terbium - 1,10 Phenanthroline (Tb-Phen) based lanthanide luminescence complex with bright green fluorescence. On addition of DPA, the green fluorescence is turn-off at a linear range from 0.6 to 4.762 mM. In this effect, 5D4- 7F5 transition caused by 1,10-phenanthroline to Tb3+ at 544 nm is restricted due to energy transfer quenching and Inner Filter Effect (IFE). The developed probe shows good sensitivity towards the detection of DPA with other coexisting biomolecules and ions with a low Limit of Detection (LOD) of 5.029 µM. The practical feasibility was evaluated in paper strip assay and extended in real samples such as human serum and tap water with satisfactory recovery percentage. Thereby, probe finds promising application in sensing of anthrax spore biomarker (DPA) and biothreat agents.

Relationship between reflectance and degree of polarization in the VNIR-SWIR: A case study on art paintings with polarimetric reflectance imaging spectroscopy.

We study the relationship between reflectance and the degree of linear polarization of radiation that bounces off the surface of an unvarnished oil painting. We design a VNIR-SWIR (400 nm to 2500 nm) polarimetric reflectance imaging spectroscopy setup that deploys unpolarized light and allows us to estimate the Stokes vector at the pixel level. We observe a strong negative correlation between the S0 component of the Stokes vector (which can be used to represent the reflectance) and the degree of linear polarization in the visible interval (average -0.81), while the correlation is weaker and varying in the infrared range (average -0.50 in the NIR range between 780 and 1500 nm, and average -0.87 in the SWIR range between 1500 and 2500 nm). By tackling the problem with multi-resolution image analysis, we observe a dependence of the correlation on the local complexity of the surface. Indeed, we observe a general trend that strengthens the negative correlation for the effect of artificial flattening provoked by low image resolutions.

Efficient nanostructured Cs2CuBr2Cl2 perovskite as a fluorescent sensor for the selective "Switch Off" detection of nitrobenzene.

Lead halide nanostructured perovskites are well known for their excellent photoluminescence and optoelectronic properties. However, lead toxicity and instability in moisture impedes its suitability for material use. Here we synthesized a highly efficient, lead free, economical, stable Cs2CuBr2Cl2 perovskite nanocrystals (PNCs) via Ligand Assisted Re-Precipitation (LARP) method which is less explored. The sensing application of the synthesized PNCs towards nitro explosives and other small organic compounds were studied. The probe exhibited high selectivity towards nitrobenzene with a lowest detection limit of 57.64 nM. The fluorescent emission intensity was drastically quenched upon the addition of 32 µM nitrobenzene. A Stern-Volmer plot was utilized for the quantification of fluorescence quenching. Further to investigate the quenching mechanism, time correlated single photon counting spectroscopy and other photoluminescence studies were performed pointing out the possibility of fluorescence resonance energy transfer. The work has been further extended to test the capability of the probe to detect nitrobenzene in real water samples and a good recovery percentage ranging from 93-98 % was obtained. Further, a paper strip assay was designed which successfully detected nitrobenzene and can be clearly noticed even with our naked eye making the probe an excellent sensor for nitrobenzene detection.

Mo(IV) Ion-Modulated BSA-Protected Gold Nanocluster Probe for Fluorescence Turn-On Detection of Trimethylamine N-Oxide (TMAO).

Trimethylamine N-oxide (TMAO), a molecule produced by the microbiota, has been associated with human health and illness. Its early discovery in body fluids may affect our understanding of the pathophysiology and treatment of many illnesses. Therefore, our knowledge of the pathophysiology and diagnostics of disorders associated with TMAO might be enhanced by the creation of dependable and fast methods for TMAO detection. Therefore, we developed a fluorescent probe for detecting TMAO utilizing an on-off-on strategy. Bovine serum albumin (BSA)@AuNCs luminescence is effectively quenched by Mo4+ because BSA@AuNCs and Mo4+ have a strong binding relationship. Mo4+ ions can substantially decrease the emission intensity of gold nanoclusters by establishing a BSA@AuNCs-Mo system. Then, the luminescence of BSA@AuNCs was restored due to the interaction between Mo4+ and TMAO. A significant linear relationship was seen between the emission intensity and TMAO concentration within the 0-201 µM range, with a detection limit of 1.532 µM. Additionally, the method can measure TMAO in blood and urine samples.

Highly sensitive lanthanide complex as a probe for l-kynurenine: A cancer biomarker.

A lanthanide complex based on europium (Eu) and chelidamic acid was synthesized (Eu-CHE) and characterized. The complex Eu-CHE exhibited intense luminescence at 615 nm under excitation at 300 nm and was further investigated for highly sensitive turn-off detection of l-kynurenine (l-kyn), a cancer biomarker. The probe detected l-kyn linearly from 6 nM to 0.2 µM with a limit of detection and limit of quantification of 1.37 and 4.57 nM, respectively. The probe was investigated for selectivity towards l-kyn among co-existing amino acids and further extended for detecting l-kyn from human serum and urine samples. A low-cost paper strip-based sensing platform was also developed for the visual detection of l-kyn.

MnO2 nanosheet quenched thulium doped photon-up conversion luminescent immunoprobe for the 'turn-on' detection of cardiac troponin T.

A "turn-on" photon up conversion nano couple based on NaYF4: Yb, Tm UCNPs quenched with MnO2 nanosheet was developed for the rapid and selective detection of cTnT. Herein, MnO2 nanosheet hold on the surface of Antibody cTnT (Ab-cTnT) conjugated blue emitting up conversion nanoprobe (λem at 475 nm), which leads to quenching of fluorescence due to energy transfer from photon up conversion nanoparticles to MnO2 nanosheets. On introducing cTnT antigen to the system, the energy transfer process is hindered due to strong antigen -antibody interface on the surface. This in turn, influences the nano-couples positions and effectively separates up conversion nanoprobe from MnO2 nanosheets surface resulting in restriction to energy transfer process enabling fluorescence recovery. The developed probe shows a linear response towards cTnT in the range of 0.16-2.77 ng/mL with a Limit of Detection (LoD) of 0.025 ng/mL. The practical feasibility of the nanoprobe is performed with possible coexisting biomolecules. Biological study in human blood serum samples exhibited sufficient recovery percentage in the range of 92-103 % is obtained.

Antibody-functionalized gold nanoclusters/gold nanoparticle platform for the fluorescence turn-on detection of cardiac troponin I.

A selective fluorescence turn-on immunosensor for the specific detection of cardiac troponin I (cTnI), the potent biomarker for myocardial infarction diagnosis, was developed with a nano couple comprised of protein-stabilized gold nanocluster and gold nanoparticle. The red fluorescence of cTnI-specific antibody tagged bovine serum albumin stabilized gold nanoclusters was quenched with gold nanoparticles (AuNP) via the intensive interaction between amine and hydroxyl functionalities of BSA and AuNP. Through this, the adsorption of gold nanoclusters at the surface of AuNP, resulting in a core-satellite assembly, was assumed to quench the fluorescence emission. While in the presence of cTnI antigen, this gets disturbed due to the formation of immunocomplex between cTnI antigen and antibody, which restricts the close interaction between gold clusters and nanoparticles, thereby restoring quenched fluorescence. The enhancement in fluorescence signal is directly related to the concentration of cTnI, and this facilitates the selective detection of cTnI in the linear concentration range 0.7 to 10 ng/mL without any interference from other potentially interfering co-existing biomolecules. An appreciable limit of detection of 0.51 ng/mL and a limit of quantification of 0.917 ng/mL for cTnI is comparable to that of the previous report.

Fluorescent Enzymatic Sensor Based Glucose Oxidase Modified Bovine Serum Albumin-Gold Nanoclusters for Detection of Glucose.

An enzymatic fluorescent probe is developed for the selective detection of glucose. In this work, a Bovine Serum Albumin stabilized gold nanocluster (BSA-AuNCs) was synthesized by microwave assisted method, and it is modified with glucose oxidase, thereby a fluorescent enzymatic sensor (BSA-AuNCs@GoX) was designed for the sensitive detection of glucose with a limit of detection of 0.03 mM. The red fluorescence exhibited by the probe is quenched by the production of H2O2 on addition of glucose via. a static quenching mechanism from UV visible absorption and Fluorescence lifetime results. The developed probe exhibits good selectivity and sensitivity with other coexisting molecular species such as glycine, creatinine, methionine, histidine, uric acid, albumin, and ions such as sodium, potassium, calcium, magnesium, zinc etc. that appear in the body fluid. The practical applicability was studied in paper strip and extended its reproducibility in biological matrixes such as human serum and urine and found a good recovery percentage of 94-101 %. By this way, we have fabricated an effective fluorescent enzymatic "turn-off" sensing probe for the detection of glucose.

L-Methionine and D-Methionine Capped Fluorescent Silicon Quantum Dots Based Probes for Turn on Sensing of Glutathione - A Comparative Study.

Oral Squamous Cell Carcinoma (OSCC), a prevalent type of oral cancer originates in squamous cells that develop due to tobacco use, excess alcohol consumption, human papillomavirus infection, chronic irritation and weakened immune system. When detected early, survival rates of OSCC can be increased to more than 85%. Hence its early detection is crucial for appropriate management. Oxidative stress has a vital role in pathogenesis of various cancers including OSCC. Early detection of OSCC can be done by exploring serum Glutathione (GSH); an oxidative stress biomarker. Herein, we have developed two Silicon quantum dots (SiQDs); (L-methionine capped Silicon quantum dots (LSiQDs) and D-methionine capped Silicon quantum dots (DSiQDs)) and their fluorescence was quenched with Cu2+. The obtained Cu@LSiQDs and Cu@DSiQDs were then explored and compared for sensing GSH. Both the SiQDs were checked for selectivity and interference studies using coexisting biomolecules extended for sensing GSH from real samples. Moreover, a paper strip assay was also developed and compared.

Picric Acid Incorporated Fluorescent Nitrogen Doped Carbon Dot for "Turn-On" Detection of Glutathione.

Glutathione (GSH), a non-protein thiol in living cells whose abnormal level indicates the onset of diseases like Alzheimer's, HIV, diabetes, cancer, Parkinson's, Dementia, etc. Herein, we have synthesized a low-cost, selective, and sensitive detection platform by using citric acid and urea-derived fluorescent carbon dots (NCDs) via the microwave-assisted method, showing fluorescence at 444 nm. This fluorescence was quenched with picric acid (PA), and this probe, picric acid incorporated nitrogen doped carbon dot (NCDs@PA) was further used for the detection of GSH. The characterization of the probe was done by photoluminescence study, UV-Visible absorption studies, ATR-FTIR, SEM, and DLS analysis. GSH induced fluorescence recovery due to the competitive binding of GSH to PA. GSH was detected within a linear range of 0.31 mM- 2.43 mM with a Limit of Detection (LoD) and Limit of Quantification (LoQ) of 32.10 µM and 107.32 µM, respectively. The sensor exhibited good selectivity and sensitivity towards GSH among various co-existing ions and biomolecules. The paper-strip-based sensing of glutathione was conducted to check practical applicability of the probe, and a real sample analysis was also conducted from spiked human samples.

NaYF4:Yb/Ho upconversion nanoprobe incorporated gold nanoparticle (AuNP) based FRET immunosensor for the "turn-on" detection of cardiac troponin I.

Cardiac troponin I (cTnI) is a significant biomarker for acute heart attack. Hence, fast, economical, easy and real time monitoring of cardiac troponin I (cTnI) is of great importance in diagnosis and prognosis of heart failure in the healthcare domain. In this work, an immunoassay based on NaYF4:Yb/Ho based photon-upconversion nanoparticle (UCNP) with narrow emission peaks at 540 nm and 655 nm respectively, is synthesized. Then, it is encapsulated with amino functionalized silica using 3-aminopropyltriethoxysilane (APTES) to form APTES@SiO2-NaYF4:Yb/Ho UCNPs. When AuNPs is added to this system, the fluorescence is quenched by the electrostatic interaction with APTES@SiO2-NaYF4:Yb/Ho UCNPs, thereby exhibiting a FRET-based biosensor. When the cTnI antigen is introduced into the developed probe, an antibody-antigen complex is formed on the surface of the UCNPs resulting in fluorescence recovery. The developed sensor shows a linear response towards cTnI in the range from 0.1693 ng mL-1 to 1.9 ng mL-1 with a low limit of detection (LOD) of 5.5 × 10-2 ng mL-1. The probe exhibits adequate selectivity and sensitivity when compared with coexisting cardiac biomarkers, biomolecules and in real human serum samples.

Logistic splicing correction for VNIR-SWIR reflectance imaging spectroscopy.

In the field of spectroscopy, a splicing correction is a process by which two spectra captured with different sensors in adjacent or overlapping electromagnetic spectrum ranges are smoothly connected. In our study, we extend this concept to the case of reflectance imaging spectroscopy in the visible-near-infrared (VNIR) and short-wave infrared (SWIR), accounting for additional sources of noise that arise at the pixel level. The proposed approach exploits the adaptive fitting of a logistic function to compute correcting coefficients that harmonize the two spectral sets. This short Letter addresses usage conditions and compares results against the existing state of the art.

Folic acid incorporated nitrogen-doped carbon dots as a turn-on fluorescence probe for homocysteine detection.

This study describes the development of a low-cost fluorescence assay for detecting homocysteine (Hcy) without the interference of cysteine and glutathione using carbon quantum dots. Herein nitrogen-doped carbon quantum dots (NCDs) were synthesized from citric acid as the carbon source and urea as the dopant using a one-pot microwave-assisted method. The obtained NCDs were incorporated with folic acid (FA) by the direct ex situ addition method and were used as a fluorescence probe to detect Hcy. The probe exhibited a fluorescence turn-on response with increased Hcy concentration up to 50 µM with a limit of detection of 2.276 µM. The point of care detection of Hcy using the probe was also tested with a paper-based assay strip.

Profiling and imaging of forensic evidence - A pan-European forensic round robin study part 1: Document forgery.

The forensic scenario, on which the round robin study was based, simulated a suspected intentional manipulation of a real estate rental agreement consisting of a total of three pages. The aims of this study were to (i) establish the amount and reliability of information extractable from a single type of evidence and to (ii) provide suggestions on the most suitable combination of compatible techniques for a multi-modal imaging approach to forgery detection. To address these aims, seventeen laboratories from sixteen countries were invited to answer the following tasks questions: (i) which printing technique was used? (ii) were the three pages printed with the same printer? (iii) were the three pages made from the same paper? (iv) were the three pages originally stapled? (v) were the headings and signatures written with the same ink? and (vi) were headings and signatures of the same age on all pages? The methods used were classified into the following categories: Optical spectroscopy, including multispectral imaging, smartphone mapping, UV-luminescence and LIBS; Infrared spectroscopy, including Raman and FTIR (micro-)spectroscopy; X-ray spectroscopy, including SEM-EDX, PIXE and XPS; Mass spectrometry, including ICPMS, SIMS, MALDI and LDIMS; Electrostatic imaging, as well as non-imaging methods, such as non-multimodal visual inspection, (micro-)spectroscopy, physical testing and thin layer chromatography. The performance of the techniques was evaluated as the proportion of discriminated sample pairs to all possible sample pairs. For the undiscriminated sample pairs, a distinction was made between undecidability and false positive claims. It was found that none of the methods used were able to solve all tasks completely and/or correctly and that certain methods were a priori judged unsuitable by the laboratories for some tasks. Correct results were generally achieved for the discrimination of printer toners, whereas incorrect results in the discrimination of inks. For the discrimination of paper, solid state analytical methods proved to be superior to mass spectrometric methods. None of the participating laboratories deemed addressing ink age feasible. It was concluded that correct forensic statements can only be achieved by the complementary application of different methods and that the classical approach of round robin studies to send standardised subsamples to the participants is not feasible for a true multimodal approach if the techniques are not available at one location.

A Robust Tensor-Based Submodule Clustering for Imaging Data Using l12 Regularization and Simultaneous Noise Recovery via Sparse and Low Rank Decomposition Approach.

The massive generation of data, which includes images and videos, has made data management, analysis, information extraction difficult in recent years. To gather relevant information, this large amount of data needs to be grouped. Real-life data may be noise corrupted during data collection or transmission, and the majority of them are unlabeled, allowing for the use of robust unsupervised clustering techniques. Traditional clustering techniques, which vectorize the images, are unable to keep the geometrical structure of the images. Hence, a robust tensor-based submodule clustering method based on l12 regularization with improved clustering capability is formulated. The l12 induced tensor nuclear norm (TNN), integrated into the proposed method, offers better low rankness while retaining the self-expressiveness property of submodules. Unlike existing methods, the proposed method employs a simultaneous noise removal technique by twisting the lateral image slices of the input data tensor into frontal slices and eliminates the noise content in each image, using the principles of the sparse and low rank decomposition technique. Experiments are carried out over three datasets with varying amounts of sparse, Gaussian and salt and pepper noise. The experimental results demonstrate the superior performance of the proposed method over the existing state-of-the-art methods.

Comparison of Imaging Models for Spectral Unmixing in Oil Painting.

The radiation captured in spectral imaging depends on both the complex light-matter interaction and the integration of the radiant light by the imaging system. In order to obtain material-specific information, it is important to define and invert an imaging process that takes into account both aspects. In this article, we investigate the use of several mixing models and evaluate their performances in the study of oil paintings. We propose an evaluation protocol, based on different features, i.e., spectral reconstruction, pigment mapping, and concentration estimation, which allows investigating the different properties of those mixing models in the context of spectral imaging. We conduct our experiment on oil-painted mockup samples of mixtures and show that models based on subtractive mixing perform the best for those materials.

Colour-Balanced Edge-Guided Digital Inpainting: Applications on Artworks.

The virtual inpainting of artworks provides a nondestructive mode of hypothesis visualization, and it is especially attractive when physical restoration raises too many methodological and ethical concerns. At the same time, in Cultural Heritage applications, the level of details in virtual reconstruction and their accuracy are crucial. We propose an inpainting algorithm that is based on generative adversarial network, with two generators: one for edges and another one for colors. The color generator rebalances chromatically the result by enforcing a loss in the discretized gamut space of the dataset. This way, our method follows the modus operandi of an artist: edges first, then color palette, and, at last, color tones. Moreover, we simulate the stochasticity of the lacunae in artworks with morphological variations of a random walk mask that recreate various degradations, including craquelure. We showcase the performance of our model on a dataset of digital images of wall paintings from the Dunhuang UNESCO heritage site. Our proposals of restored images are visually satisfactory and they are quantitatively comparable to state-of-the-art approaches.

Folic Acid as a Bimodal Optical Probe for the Detection of TNT.

Rapid and onsite detection of nitroaromatic explosive 2,4,6-trinitrotoluene (TNT) is very crucial for the safety and security of human life as well as for the environment. In this present work, we demonstrate the feasibility for employing Folic Acid (FA) as a fluorescent as well as a colorimetric probe for the detection of TNT. This probe was synthesized by a simple one-step process. The developed probe shows an emission maximum at 490 nm upon excitation at 420 nm. On adding TNT, the fluorescence of the FA probe is quenched. Also, it shows a good selectivity towards TNT over other similar organic compounds such as 4-nitrophenol (4-NP), 2,4-dinitrophenol (2,4-DNP) and picric acid (PA). The limit of detection (LoD) of TNT was found to be 1.9398 µM. Colorimetric detection was conducted and paper strip assay was developed for the practical applications.