The gold-nanoparticle-based surface plasmon resonance light scattering and visual DNA aptasensor for lysozyme

Lysozyme-sensitive plasmonic hydrogel nanocomposite for colorimetric dry-eye inflammation biosensing

Detection of lysozyme levels in ocular fluids is considered crucial for diagnosing and monitoring various health and eye conditions, including dry-eye syndrome. Hydrogel-based nanocomposites have been demonstrated to be one of the most promising platforms for fast and accurate sensing of different biomolecules. In this work, hydrogel, electrospun nanofibers, and plasmonic nanoparticles are combined to fabricate a sensitive and easy-to-use biosensor for lysozyme. Poly(L-lactide-co-caprolactone) (PLCL) nanofibers were covered with silver nanoplates (AgNPls), providing a stable plasmonic platform, where a poly(N-isopropylacrylamide)-based (PNIPAAm) hydrogel layer allows mobility and good integration of the biomolecules. By integrating these components, the platform can also exhibit a colorimetric response to the concentration of lysozyme, allowing for easy and non-invasive monitoring. Quantitative biosensing operates on the principle of localized surface plasmon resonance (LSPR) induced by plasmonic nanoparticles. Chemical, structural, thermal, and optical characterizations were performed on each platform layer, and the platform's ability to detect lysozyme at concentrations relevant to those found in tears of patients with dry-eye syndrome and other related diseases was investigated by colorimetry and UV-Vis spectroscopy. This biosensor's sensitivity and rapid response time, alongside the easy detection by the naked eye, make it a promising tool for early diagnosis and treatment monitoring of eye diseases.

Stoichiometric Tuning of PNA Probes to Au0.8Ag0.2 Alloy Nanoparticles for Visual Detection of Nucleic Acids in Plasma

Standard detection methods for nucleic acids, an important class of diagnostic biomarkers, are often laborious and cumbersome. In need for development of facile methodologies, localized surface plasmon resonance (LSPR) assays have been widely explored for both spectroscopic and visual detection of nucleic acids. Our sensing approach is based on monitoring changes in the LSPR band due to interaction between peptide nucleic acid (PNA) and plasmonic nanoparticles (NPs) in the presence/absence of target nucleic acid. We have investigated the importance of tuning the stoichiometry of PNA to NPs to enable "naked-eye" detection of nucleic acids at clinically relevant concentration ranges. Assaying in plasma is achieved by incorporation of silver in gold NPs (AuNPs) via an alloying process. The synthesized gold/silver alloy NPs reduce nonspecific adsorption of proteinaceous interferents in plasma. Furthermore, the gold/silver alloy NPs absorb in the most sensitive cyan to green transition zone (∼500 nm) yielding highly competitive visual limits of detection (LODs). The visual LOD (calculated objectively using the ΔE algorithm) for a model microRNA (mir21) using a productive combination of stoichiometric tuning of the PNA to NP ratio and compositional tuning of the NPs in buffer and plasma extract equals 200 pM (∼250 times lower than existing reports) and 3 nM, respectively. We envision that the proposed LSPR assay based on Au_0.8Ag_0.2NPs offers an avenue for rapid and sensitive on-site detection of nucleic acids in complex matrixes in combination with efficient target extraction kits.

Colorimetric Urinalysis for On-Site Detection of Metabolic Biomarkers

Over the past few decades, colorimetric assays have been developed for cost-effective and rapid on-site urinalysis. Most of these assays were employed for detection of biomarkers such as glucose, uric acid, ions, and albumin that are abundant in urine at micromolar to millimolar levels. In contrast, direct assaying of urinary biomarkers such as glycated proteins, low-molecular-weight reactive oxygen species, and nucleic acids that are present at significantly lower levels (nanomolar to picomolar) remain challenging due to the interferences from the urine sample matrix. State-of-the-art assays for detection of trace amounts of urinary biomarkers typically utilize time-consuming and equipment-dependent sample pretreatment or clean-up protocols prior to assaying, which limits their applicability for on-site analysis. Herein, we report a colorimetric assay for on-site detection of trace amount of generic biomarkers in urine without involving tedious sample pretreatment protocols. The detection strategy is based on monitoring the changes in optical properties of poly(3-(4-methyl-3'-thienyloxy)propyltriethylammonium bromide) upon interacting with an aptamer or a peptide nucleic acid in the presence and absence of target biomarkers of relevance for the diagnosis of metabolic complications and diabetes. As a proof of concept, this study demonstrates facile assaying of advanced glycation end products, 8-hydroxy-2'-deoxyguanosine and hepatitis B virus DNA in urine samples at clinically relevant concentrations, with limits of detection of ∼850 pM, ∼650 pM, and ∼ 1 nM, respectively. These analytes represent three distinct classes of biomarkers: (i) glycated proteins, (ii) low-molecular-weight reactive oxygen species, and (iii) nucleic acids. Hence, the proposed methodology is applicable for rapid detection of generic biomarkers in urine, without involving sophisticated equipment and skilled personnel, thereby enabling on-site urinalysis. At the end of the contribution, we discuss the opportunity to translate the homogeneous assay into a paper-based format.

Aptamer-functionalized magnetic metal organic framework as nanoprobe for biomarkers in human serum

Human serum is a huge bioinformatics database of human physiological and pathological state, many proteins/peptides among which can serve as biomarkers for monitoring human's health condition, thereby being worth exploring. The simple and fast capture of biomarkers from human serum is the first key step to realize their accurate detection. In this work, we developed the aptamer functionalized magnetic metal organic framework nanoprobe, and furtherly combined with mass spectrometry technology to establish an efficient method of identifying biomarkers. Taking insulin as example of biomarker in human serum, we developed sulfhydryl human insulin aptamer functionalized magnetic metal organic framework (denoted as Mag MOF@Au@HIA) through the post-synthetic modification of MIL-101(Cr)-NH₂ for testing the applicability of the established method. Depending on the strong magnetic responsiveness and high specific area as well as high-loaded human insulin aptamers, the limit of detection of insulin was down to 1 ng/mL and 2 ng/mL in the standard insulin solution and human serum, respectively. Moreover, a good linear relationship (R² = 0.998) was obtained by using standard insulin solution with concentration range from 100 ng/mL to 5 ng/mL, based on which the capture recovery of insulin with Mag MOF@Au@HIA from human serum was demonstrated to be excellent. All of the results indicate that the aptamer-functionalized magnetic metal organic framework is a promising nanoprobe for biomarkers capture in human serum.

Selective and sensitive detection of lysozyme based on plasmon resonance light-scattering of hydrolyzed peptidoglycan stabilized-gold nanoparticles

The simple, economic, rapid, and sensitive detection of lysozyme has an important significance for disease diagnosis since it is a potential biomarker. In this work, a new detection strategy for lysozyme was developed based on the change of the plasmon resonance light scattering (PRLS) signal of peptidoglycan stabilized gold nanoparticles (PGN-AuNPs). Peptidoglycan (PGN) was employed as a stabilizer to prepare PGN-AuNPs which have the properties of a uniform particle size, good stability, and a specific biological function. Due to the specific cleavage of lysozyme to PGN, a very simple specific and sensitive detection method for lysozyme was developed based on the PRLS signal of PGN-AuNPs after mixing with lysozyme for 1.5 h. The enhanced PRLS signals (ΔI_PRLS, at 560 nm) increased linearly with increasing lysozyme in the range 5 nM to 1600 nM with the detection limit down to 2.32 nM (ΔI_PRLS = 41.6397 + 0.5332c, R = 0.9961). When the PGN-AuNP based method was applied to assay lysozyme in authentic human serum samples, the recovery efficiency was 106.76-119.32% with the relative standard deviations in the range of 0.14-3.11%, showing good feasibility. The PGN-AuNP based method for lysozyme assay developed here is simple, rapid, selective, and sensitive, which is expected to provide a feasible new method for the diagnosis or prognosis of lysozyme-related diseases in a clinical setting.

Sensitive colorimetric detection for lysozyme based on the capture of a fixed thiol-aptamer on gold nanoparticles

A thiol-aptamer immobilized on gold nanoparticles enhances the stability of probes for detecting lysozyme with a LOD of 0.054 nM.

A facile electrochemical aptasensor for lysozyme detection based on target-induced turn-off of photosensitization

The quantification of proteins is essential in fundamental research or clinical applications. Here, we developed a facile electrochemical aptasensor based on target-induced turn-off of photosensitization for label-free and ultrasensitive detection of protein (exemplified by lysozyme). EB (ethidium bromide) molecules that were embedded in dsDNA between lysozyme binding aptamer and complementary DNA immobilized on the electrode, could photo-cleave the dsDNA via singlet oxygen (O₂¹) during photosensitization, resulting in a high voltammetry current of the [Fe(CN)₆]^3-/4-. Upon recognition of the lysozyme by aptamer, the EB molecules were released from dsDNA, and its photosensitization activity was turned off. As a result, more amount of complementary DNA was retained on the Au nanoparticles modified carbon nanotube paste electrode (AuNPs-CNPE), leading to a declined voltammetry current. Such a sensing strategy allowed detection of 10 pM-1 µM lysozyme with a low detection limit (about 2 pM). Besides, the sensor was free of labeling procedure as well as extra signal amplification step, and the CNPE modification was quite simple, only with AuNPs. The sensor also showed excellent selectivity toward lysozyme in the presence of interfering proteins, such as thrombin, bovine serum albumin, myoglobin, etc. The proposed sensor was applied to the determination of lysozyme in urine samples with the recoveries ranging from 96.6% to 101%. The proposed biosensor holds a great promise in developing other electrochemical sensors based on photosensitization.

A magnetic nanoparticle-based aptasensor for selective and sensitive determination of lysozyme with strongly scattering silver nanoparticles

Qualitative and quantitative determination of lysozyme concentrations in urine and serum with high selectivity and sensitivity is important for diagnosing the progression of several diseases. In this report, we devised an improved method for specifically detecting lysozyme by combining magnetic nanoparticles (for separation and enrichment), an aptamer (for selective binding of lysozyme) and strongly scattering silver nanoparticles (AgNPs, for detection by light scattering, but also providing another level of selectivity due to their electrostatic binding with lysozyme). In this system, 0.4-30 nM lysozyme could be simply detected owing to the decreased light scattering of AgNPs in solution after magnetic separation, with a detection limit of 100 pM. In addition, lysozyme was also able to be semi-quantified by using the dark-field light scattering images of AgNPs after enrichment by the MNP-apt-lysozyme complex. Moreover, this design shows great promise for the robust and reliable detection of lysozyme in real samples, with a recovery rate ranging from 98.6% to 101.3% in human serum samples. Therefore, this assay provided robust measurements with good specificity, sensitivity, and tolerance of changes in the sample matrix. We expect that this MNP-based aptasensor may find utility in the accurate diagnosis of lysozyme-related diseases.

Gold nanoparticle-based colorimetric biosensors

Gold nanoparticles (AuNPs) provide excellent platforms for the development of colorimetric biosensors as they can be easily functionalised, displaying different colours depending on their size, shape and state of aggregation. In the last decade, a variety of biosensors have been developed to exploit the extent of colour changes as nano-particles (NPs) either aggregate or disperse, in the presence of analytes. Of critical importance to the design of these methods is that the behaviour of the systems has to be reproducible and predictable. Much has been accomplished in understanding the interactions between a variety of substrates and AuNPs, and how these interactions can be harnessed as colorimetric reporters in biosensors. However, despite these developments, only a few biosensors have been used in practice for the detection of analytes in biological samples. The transition from proof of concept to market biosensors requires extensive long-term reliability and shelf life testing, and modification of protocols and design features to make them safe and easy to use by the population at large. Developments in the next decade will see the adoption of user friendly biosensors for point-of-care and medical diagnosis as innovations are brought to improve the analytical performances and usability of the current designs. This review discusses the mechanisms, strategies, recent advances and perspectives for the use of AuNPs as colorimetric biosensors.

Effects of Cationic Proteins on Gold Nanoparticle/Aptamer Assays

Gold nanoparticles (AuNPs) and aptamers are compelling building blocks for analytical assays with desired attributes of selectivity and sensitivity and may theoretically form the basis of instrument-free color-changing assays for any target against which a DNA aptamer has been selected. However, assays for proteins based on these components may be subject to significant interferences from the interaction of proteins with nanoparticles. We found that for three representative protein/aptamer systems-thrombin, apolipoprotein E, and platelet-derived growth factor-pH-dependent aggregation occurred, even in the absence of the aptamer, to differing extents. This effect is most pronounced when proteins display net surface charge (i.e., when pH < pI) but can even be observed at pH = pI when the protein retains regions of positive charge. These interactions of AuNPs and cationic regions on proteins may present an important limitation on the development of AuNP-based analytical assays.

Aptamer-Based SERS Detection of Lysozyme on a Food-Handling Surface

Undeclared food allergens due to cross contamination of processing equipment is a leading cause for food product recalls. Therefore, there is a great need for developing rapid and sensitive methods to detect food allergens. In this paper, an aptamer highly specific to egg white lysozyme was coupled to dendritic silver nanoparticles in order to perform surface enhanced Raman spectroscopy (SERS). The procedure was successfully tested in water and on a stainless steel food-handling surface. The lowest detectable concentration for lysozyme was 0.5 μg/mL in water and 5 μg/mL on a stainless steel food-handling surface. Principal component analysis shows a significant change in SERS spectra when lysozyme was present, suggesting the successful capture of lysozyme by the aptamer. Quantification of lysozyme target was also shown from 0 to 6 μg/mL, that is, 0, 0.5, 2, 6 μg/mL. Overall method took less than 40 min. The developed method can be extended to detect other food allergens using specific aptamers.

Nanoparticles and DNA - a powerful and growing functional combination in bionanotechnology

Functionally integrating DNA and other nucleic acids with nanoparticles in all their different physicochemical forms has produced a rich variety of composite nanomaterials which, in many cases, display unique or augmented properties due to the synergistic activity of both components. These capabilities, in turn, are attracting greater attention from various research communities in search of new nanoscale tools for diverse applications that include (bio)sensing, labeling, targeted imaging, cellular delivery, diagnostics, therapeutics, theranostics, bioelectronics, and biocomputing to name just a few amongst many others. Here, we review this vibrant and growing research area from the perspective of the materials themselves and their unique capabilities. Inorganic nanocrystals such as quantum dots or those made from gold or other (noble) metals along with metal oxides and carbon allotropes are desired as participants in these hybrid materials since they can provide distinctive optical, physical, magnetic, and electrochemical properties. Beyond this, synthetic polymer-based and proteinaceous or viral nanoparticulate materials are also useful in the same role since they can provide a predefined and biocompatible cargo-carrying and targeting capability. The DNA component typically provides sequence-based addressability for probes along with, more recently, unique architectural properties that directly originate from the burgeoning structural DNA field. Additionally, DNA aptamers can also provide specific recognition capabilities against many diverse non-nucleic acid targets across a range of size scales from ions to full protein and cells. In addition to appending DNA to inorganic or polymeric nanoparticles, purely DNA-based nanoparticles have recently surfaced as an excellent assembly platform and have started finding application in areas like sensing, imaging and immunotherapy. We focus on selected and representative nanoparticle-DNA materials and highlight their myriad applications using examples from the literature. Overall, it is clear that this unique functional combination of nanomaterials has far more to offer than what we have seen to date and as new capabilities for each of these materials are developed, so, too, will new applications emerge.

Photoluminescence Mechanism of DNA-Templated Silver Nanoclusters: Coupling between Surface Plasmon and Emitter and Sensing of Lysozyme

DNA-templated silver nanoclusters (DNA-AgNCs) have now been thrust into the limelight with their superior optical properties and potential biological applications. However, the origin of photoluminescence from DNA-AgNCs still remains unclear. In this work, DNA-AgNCs were synthesized and the photoluminescence properties as well as the biosensing applications of the designed DNA-AgNCs were investigated. The photoluminescence properties of the DNA-AgNCs were studied under three regions of excitation wavelength based on the UV-visible absorption spectra. It was deemed that the photoluminescence originated from coupling between the surface plasmon and the emitter in AgNCs when they were excited by visible light above 500 nm, and thus the emission wavelength varied with changing the excitation wavelength. The photoluminescence of the red-emitting-only AgNCs was the intrinsic fluorescence when excited from 200 to 400 nm, which was only related to the emitter; but for two components of blue- and red-emitting AgNCs, the emission wavelength varied with the excitation wavelength ranging from 300 to 360 nm, and the photoluminescence was a coupling between the surface plasmon and the emitter. The photoluminescence was only related to the surface plasmon when the AgNCs were excited from 400 to 500 nm. Four DNA probes were designed and each contained two parts: one part was the template used to synthesize AgNCs and it was same to all, and the other part was the lysozyme binding DNA (LBD) used to bind lysozyme and two kinds of LBD were studied. It was deemed that the difference in DNA bases, sequence, and secondary structure caused the synthesized DNA-AgNCs to be different in photoluminescence properties and sensing ability to lysozyme, and the sensing mechanism based on photoluminescence enhancement was also presented. This work explored the origin of photoluminescence and the sensing ability of DNA-AgNCs, and is hoped to make a better understanding of this kind of photoluminescence probe.

Detection of urinary creatinine using gold nanoparticles after solid phase extraction

Label-free gold nanoparticles (AuNPs) were utilized in the detection of creatinine in human urine after a sample preparation by extraction of creatinine on sulfonic acid functionalized silica gel. With the proposed sample preparation method, the interfering effects of the urine matrix on creatinine detection by AuNPs were eliminated. Parameters affecting creatinine extraction were investigated. The aggregation of AuNPs induced by creatinine resulted in a change in the surface plasmon resonance signal with a concomitant color change that could be observed by the naked eye and quantified spectrometrically. The effect of AuNP concentration and reaction time on AuNP aggregation was investigated. The method described herein provides a determination of creatinine in a range of 15-40mgL(-1) with a detection limit of 13.7mgL(-1) and it was successfully used in the detection of creatinine in human urine samples.

Plasmonic nanomaterials for biodiagnostics

The application of nanomaterials to detect disease biomarkers is giving rise to ultrasensitive assays, with scientists exploiting the many advantageous physical and chemical properties of nanomaterials. The fundamental basis of such work is to link unique phenomena that arise at the nanoscale to the presence of a specific analyte biomolecule, and to modulate the intensity of such phenomena in a ratiometric fashion, in direct proportion with analyte concentration. Precise engineering of nanomaterial surfaces is of utmost importance here, as the interface between the material and the biological environment is where the key interactions occur. In this tutorial review, we discuss the use of plasmonic nanomaterials in the development of biodiagnostic tools for the detection of a large variety of biomolecular analytes, and how their plasmonic properties give rise to tunable optical characteristics and surface enhanced Raman signals. We put particular focus on studies that have explored the efficacy of the systems using physiological samples in an effort to highlight the clinical potential of such assays.

A new and highly sensitive resonance Rayleigh scattering assay for lysozyme using aptamer-nanogold as a probe

Gold nanoparticles (GN), 10 nm in size, were modified by using lysozyme aptamer (Apt) to obtain a stable Apt–GN probe in pH 8.05 Tris/HCl buffer solutions containing 0.04 mol/L NaCl. Upon addition of lysozyme (LYS), it reacted with the Apt of the probe to form a very stable Apt–LYS complex and to release GNs, which aggregated to form large clusters with a resonance Rayleigh scattering (RRS) peak at 368 nm. The enhanced peak intensity, ΔI, was linear to the LYS concentration in the range 0.2–5.2 nmol/L, with a detection limit of 0.05 nmol/L. The influence of foreign substance was tested, and the results showed that this RRS method has high selectivity. This Apt–GN RRS method was applied to the analysis of LYS in a real sample, with satisfactory results.

A highly sensitive dual-readout assay based on poly(A) and gold nanoparticles for palmatine hydrochloride

This report presents a highly sensitive, poly(A)-stabilized gold nanoparticle-based assay with dual readouts (resonance light scattering and colorimetric) for detecting palmatine hydrochloride (PaH) in real samples. The detection mechanism is based on the fact that palmatine hydrochloride has strong affinity to poly(A), which can stabilize gold nanoparticles at high ionic strength, and cause the aggregation of poly(A)-stabilized AuNPs, resulting in the enhanced resonance light scattering (RLS). At the same time, the color change of poly(A)-stabilized AuNPs solution is from red to blue via purple. Thus a highly sensitive RLS assay for PaH has been developed with a linear range of 0.023-2.5 μg/mL. The limit of detection (LOD, 3σ) is 2.3 ng/mL. In this work, the reaction mechanism of this system was investigated by scanning electron microscope (SEM), dark-field light scattering images (DLSI), dynamiclight scattering (DLS) and circular dichroism (CD). This proposed method was also applied successfully for the determination of PaH in pharmaceutical preparations and urine samples with RSD⩽4.0%. The results are in good agreement with those from the official method.

A tailor-made nucleoside-based colourimetric probe of formic acid

A ratiometric, specific probe of formic acid has been developed.

Tuning of the near-infrared localized surface plasmon resonance of Cu2−xSe nanoparticles with lysozyme-induced selective aggregation

Poly(styrene sulfonic acid)sodium stabilized Cu_2−xSe nanoparticles (PSS–Cu_2−xSe NPs) with localized surface plasmon resonance (LSPR) absorption centered at 980 nm can be selectively aggregated by lysozyme (Lys) through the electrostatic attraction, giving rise to a red shift of the LSPR in the near-infrared (NIR) region.

Development of a label-free aptasensor for monitoring the self-association of lysozyme

A novel aptamer and surface plasmon resonance (SPR)-based sensor was developed for the label-free detection of lysozyme. The aptasensor is characterised by a detection limit of 1 μg mL(-1) and a linear range of 5-50 μg mL(-1). As an application, we examined the usefulness of the aptasensor for monitoring the early stages of the aggregation of lysozyme. It was surprisingly found that, despite a significant decrease in monomer content during aggregation, the response of the aptasensor for protein solutions aged for 12 hours was similar to that for the fresh protein. To correlate the results obtained with the aptasensor with the composition of lysozyme solutions at various time points, we examined them in detail by atomic force microscopy (AFM), thioflavin T fluorescence, size-exclusion chromatography (SEC) and Matrix Assisted Laser Desorption Ionisation Time of Flight Mass Spectrometry (MALDI-TOF-MS). All methods together indicated that during the initial hours of aggregation, the protein solutions contained small lysozyme oligomers (mainly dimers) and decreasing amounts of monomers. Our results thus suggest that the aptamer also recognizes lysozyme dimers/oligomers. A higher non-specific binding was observed for the aggregated lysozyme at the surface of the aptasensor as compared to the native protein. This was attributed to the hydrophobic patches which are exposed by the unfolded lysozyme and/or oligomer species, allowing for different adsorption and organisation at the surface of the aptasensor. This hypothesis is supported by square wave voltammetry (SWV) studies using solutions of aggregated lysozyme. A higher electrochemical signal due to the direct oxidation of tyrosine/tryptophan residues was observed for aged protein solutions as compared to the fresh solution, indicative of an increased number of such exposed electroactive residues and of overall increased surface hydrophobicity of the protein. Our work presents a label-free lysozyme aptasensor that is useful not only for the detection of the protein monomer but also for observing the onset of aggregation. The approach can be extended to other proteins which are prone to aggregation.

Mechanistic investigation of seeded growth in triblock copolymer stabilized gold nanoparticles

We report the seeded synthesis of gold nanoparticles (GNPs) via the reduction of HAuCl4 by (L31 and F68) triblock copolymer (TBP) mixtures. In the present study, we focused on [TBP]/[Au(III)] ratios of 1-5 (≈1 mM HAuCl4) and seed sizes ~20 nm. Under these conditions, the GNP growth rate is dominated by both the TBP and seed concentrations. With seeding, the final GNP size distributions are bimodal. Increasing the seed concentration (up to ~0.1 nM) decreases the mean particle sizes 10-fold, from ~1000 to 100 nm. The particles in the bimodal distribution are formed by the competitive direct growth in solution and the aggregative growth on the seeds. By monitoring kinetics of GNP growth, we propose that (1) the surface of the GNP seeds embedded in the TBP cavities form catalytic centers for GNP growth and (2) large GNPs are formed by the aggregation of GNP seeds in an autocatalytic growth process.

Self-assembly of folate onto polyethyleneimine-coated CdS/ZnS quantum dots for targeted turn-on fluorescence imaging of folate receptor overexpressed cancer cells

Folate receptor (FR) can be overexpressed by a number of epithelial-derived tumors, but minimally expressed in normal tissues. As folic acid (FA) is a high-affinity ligand to FR, and not produced endogenously, development of FA-conjugated probes for targeted imaging FR overexpressed cancer cells is of significance for assessing cancer therapeutics and for better understanding the expression profile of FR in cancer. Here we report a novel turn-on fluorescence probe for imaging FR overexpressed cancer cells. The probe was easily fabricated via electrostatic self-assembly of FA and polyethyleneimine-coated CdS/ZnS quantum dots (PEI-CdS/ZnS QDs). The primary fluorescence of PEI-CdS/ZnS QDs turned off first upon the electrostatic adsorption of FA onto PEI-CdS/ZnS QDs based on electron transfer to produce negligible fluorescence background. The presence of FR expressed on the surface of cancer cells then made FA desorb from PEI-CdS/ZnS QDs due to specific and high affinity of FA to FR. As a result, the primary fluorescence of PEI-CdS/ZnS QDs adhering to the cells turned on due to the inhibition of electron transfer. The most important merits of the developed probe are its simplicity and the effective avoidance of the false positive results due to the simple electrostatic self-assembly of FA onto the surface of PEI-CdS/ZnS QDs and the involved fluorescence "off-on" mechanism. The probe was demonstrated to be sensitive and selective for targeted imaging of FR overexpressed cancer cells in turn-on mode.

Assays for aptamer-based platforms

Aptamers are single stranded DNA or RNA oligonucleotides that have high affinity and specificity towards a wide range of target molecules. Aptamers have low molecular weight, amenable to chemical modifications and exhibit stability undeterred by repetitive denaturation and renaturation. Owing to these indispensable advantages, aptamers have been implemented as molecular recognition element as alternative to antibodies in various assays for diagnostics. By amalgamating with a number of methods that can provide information on the aptamer-target complex formation, aptamers have become the elemental tool for numerous biosensor developments. In this review, administration of aptamers in applications involving assays of fluorescence, electrochemistry, nano-label and nano-constructs are discussed. Although detection strategies are different for various aptamer-based assays, the core of the design strategies is similar towards reporting the presence of specific target binding to the corresponding aptamers. It is prognosticated that aptamers will find even broader applications with the development of new methods of transducing aptamer target binding.

The homogeneous fluorescence anisotropic sensing of salivary lysozyme using the 6-carboxyfluorescein-labeled DNA aptamer

A simple and sensitive fluorescence anisotropy method was developed for lysozyme, employing the coupling of fluorophore, 6-carboxyfluorescein (FAM), with lysozyme upon recognition between the target molecule and its DNA aptamer. It was found in this study that the rotational dynamic of the detecting system is crucial to obtain a high anisotropy signal that cannot always be achieved by simply increasing the molecular volume, because molecular volume increase may not be able to efficiently retard the rotational movement of the fluorophore. FAM was selected as the label of the ssDNA aptamer to effectively facilitate the change of the fluorophore from a primarily independent segmental movement to slow global rotation. The time-resolved measurements, including lifetime and dynamic fluorescence anisotropy, were conducted to study the recognition interaction and to better understand the methodology. The proposed method had a wide linear dynamic range of 12.5-300 nM and a high sensitivity with the limit of detection of 4.9 nM (3S/N). This proposed method was successfully applied to assay of human salivary lysozyme. The results based on the standard addition recovery and comparison with enzyme-linked immunosorbent assay (ELISA) demonstrated the feasibility of this method for biological samples. Using coupling between the fluorophore and the analyte can be one of the approaches working toward expanding the application of fluorescence anisotropy based on aptamer-target and antibody-antigen recognitions.