EGaIn-Modified ePADs for Simultaneous Detection of Homocysteine and C-Reactive Protein in Saliva toward Early Diagnosis of Cardiovascular Disease

Homocysteine (Hcy) and C-reactive protein (CRP) are critical biomarkers for numerous chronic diseases, with cardiovascular disease (CVD) being the most prevalent. The ability to simultaneously detect both biomarkers in point-of-care settings is in high demand for CVD early diagnosis and prevention. Herein, we prepared the eutectic gallium indium (EGaIn) nanoparticles decorated with p-phenylenediamine (PPD) on the surface to facilitate the subsequent attachment of gold nanoparticles (AuNPs) to achieve EGaIn-PPD@Au, which was modified on the screen-printed electrochemical paper-based analytical devices (ePADs). Aptamers that are specific to Hcy and CRP were then immobilized on the EGaIn-PPD@Au surface to achieve the sensing interface on ePADs. The presence of EGaIn-PPD@Au significantly enhanced the electrical conductivity, leading to amplified electrochemical signals. This aptasensor demonstrated high specificity, capable of detecting Hcy in a range of 1-50 μM with a detection limit of 0.22 μM, and the detection range for CRP was 1-100 ng/mL with a detection limit of 0.039 ng/mL. The aptasensor also effectively detected Hcy and CRP in clinical saliva samples, yielding an area under the curve (AUC) of about 0.80 when the individual biomarker was considered and 0.93 when both biomarkers were taken into account. The positive correlation observed between salivary and blood concentrations of Hcy and CRP, coupled with their association with cardiovascular disease (CVD), suggested the potential of this methodology as a noninvasive point-of-care strategy for the early diagnosis of CVD.

A lateral flow strip for on-site detection of homocysteine based on a truncated aptamer

An elevated level of homocysteine (Hcy) in serum is closely related to the development of various diseases. Therefore, homocysteine has been widely employed as a biomarker in medical diagnosis and the on-site detection of homocysteine is highly desired. In this study, a truncated highly specific aptamer for homocysteine was screened and used to design a lateral flow strip (LFS) for the detection of homocysteine. The aptamer was derived from a previously reported sequence. Based on the result of molecular docking, the original sequence was subjected to truncation, resulting in a reduction of the length from 66 nt to 55 nt. Based on the truncated aptamer, the LFS was designed for the detection of homocysteine. In the presence of homocysteine, the aptamer selectively binds to it, releasing cDNA from the aptamer/cDNA duplex. This allows cDNA to bind to the capture probe immobilized on the T zone of the strip, resulting in a red signal on the T zone from gold nanoparticles (AuNPs). The strip enables the visual detection of homocysteine in 5 min. Quantitative detection can be facilitated with the aid of ImageJ software. In this mode, the linear detection range for homocysteine is within 5-50 μM, with a detection limit of 4.18 μM. The strip has been effectively utilized for the detection of homocysteine in human serum. Consequently, the combination of the truncated aptamer and the strip offers a method that is sensitive, quick, and economical for the on-site detection of homocysteine.

Selection of regioselective DNA aptamer for detection of homocysteine in nondeproteinized human plasma

Small molecule-binding aptamers often suffer from high cross reactivity to structure analogues in biological samples, limiting their value for clinical diagnosis. Herein, we present a method to overcome this issue, by performing binding-inhibited organic reaction-based regioselective selection of aptamers against homocysteine (Hcy), which is a marker for diagnosing many disorders including stroke and Alzheimer's. This approach has led to isolation of a DNA aptamer that binds to the alkane thiol chain of Hcy with exceptional specificity against cysteine. It also binds with oxidized Hcy at weaker affinity. Using this new aptamer, we produced a reusable fluorescent optical fiber aptasensor for direct and validated detection of both free and total Hcy in nondeproteinized patient plasma in the diagnostic concentration range. The binding site-specific aptamer selection and optical-fiber-sensing strategy can expand the practical utility of aptamers in clinical diagnosis.

A one-step aptasensor for ultrasensitive detection of lung cancer marker homocysteine based on multifunctional carbon nanotubes by square-wave voltammetry

In this work, a one-step aptasensor for ultrasensitive detection of homocysteine (HCY) is developed based on multifunctional carbon nanotubes, which is magnetic multi-walled carbon nanotubes (Fe₃O₄@MWCNTs) combined with the aptamer (Apt) for HCY (Fe₃O₄@MWCNTs-Apt). Fe₃O₄@MWCNTs-Apt have multiple functions as follows. (1) Apt immobilized could selectively capture all target molecules HCY in the sample; (2) Magnetic Fe₃O₄ nanoparticles could separate all target molecules HCY captured by Apt from the sample substrate to eliminate the background interference and achieve one-step preparation of the aptasensor; And (3), MWCNTs with good electrical conductivity become a new electrode surface, construct a three-dimensional electrode surface network, make the electron transfer easier and thus then enhance the signal response. Results show that there is a good linear relationship between peak current of square-wave voltammetry (SWV) and HCY concentration in the range of 0.01 μmol/L-1 μmol/L, with a limit of detection (LOD) 0.002 μmol/L. And, selectivity, reproducibility, precision and accuracy are all satisfactory. In addition, it could be applied to the detection of HCY in the plasma of lung cancer patients successfully, suggesting that this one-step aptasensor for HCY has a potential in practical clinical applications.

A Label-Free Electrochemical Biosensor for Homocysteine Detection Using Molecularly Imprinted Polymer and Nanocomposite-Modified Electrodes

An essential biomarker for the early detection of cardiovascular diseases is serum homocysteine (Hcy). In this study, a molecularly imprinted polymer (MIP) and nanocomposite were used to create a label-free electrochemical biosensor for reliable Hcy detection. A novel Hcy-specific MIP (Hcy-MIP) was synthesized using methacrylic acid (MAA) in the presence of trimethylolpropane trimethacrylate (TRIM). The Hcy-MIP biosensor was fabricated by overlaying the mixture of Hcy-MIP and the carbon nanotube/chitosan/ionic liquid compound (CNT/CS/IL) nanocomposite on the surface of a screen-printed carbon electrode (SPCE). It showed high sensitivity, with a linear response of 5.0 to 150 µM (R² of 0.9753) and with a limit of detection (LOD) at 1.2 µM. It demonstrated low cross-reactivity with ascorbic acid, cysteine, and methionine. Recoveries of 91.10-95.83% were achieved when the Hcy-MIP biosensor was used for Hcy at 50-150 µM concentrations. The repeatability and reproducibility of the biosensor at the Hcy concentrations of 5.0 and 150 µM were very good, with coefficients of variation at 2.27-3.50% and 3.42-4.22%, respectively. This novel biosensor offers a new and effective method for Hcy assay compared with the chemiluminescent microparticle immunoassay at the correlation coefficient (R²) of 0.9946.

Fabrication of a Novel and Ultrasensitive Label-Free Electrochemical Aptasensor Based on Gold Nanostructure for Detection of Homocysteine

The current attempt was made to detect the amino acid homocysteine (HMC) using an electrochemical aptasensor. A high-specificity HMC aptamer was used to fabricate an Au nanostructured/carbon paste electrode (Au-NS/CPE). HMC at high blood concentration (hyperhomocysteinemia) can be associated with endothelial cell damage leading to blood vessel inflammation, thereby possibly resulting in atherogenesis leading to ischemic damage. Our proposed protocol was to selectively immobilize the aptamer on the gate electrode with a high affinity to the HMC. The absence of a clear alteration in the current due to common interferants (methionine (Met) and cysteine (Cys)) indicated the high specificity of the sensor. The aptasensor was successful in sensing HMC ranging between 0.1 and 30 μM, with a narrow limit of detection (LOD) as low as 0.03 μM.

A two-dimensional thin Co-MOF nanosheet as a nanozyme with high oxidase-like activity for GSH detection

A two-dimensional thin Co-MOF (ZIF-67) nanosheet with high oxidase-like activity was applied for sensitive visual GSH detection.

Heat-enhancing aggregation of gold nanoparticles combined with loop-mediated isothermal amplification (HAG-LAMP) for Plasmodium falciparum detection

Malaria infection represents a major public health and economic issue that leads to morbidity and mortality globally. A highly effective and uncomplicated detection tool is required for malaria control in geographical hotspots of transmission. We developed a simple and more sensitive novel approach for the detection of the 18S rRNA gene of Plasmodium falciparum based on loop-mediated isothermal amplification (LAMP) and visualization using colorimetric, streptavidin-functionalized gold nanoparticles (SA-GNPs). Two loop primers of LAMP were biotinylated to produce biotin-containing products during amplification. After the addition of SA-GNPs, clusters of avidin-biotin complexes were established in the LAMP structure. While the positive reactions remained wine red, the negative reactions became colorless with partial aggregations induced by hydrochloric acid (HCl) under heat enhancement (60 °C). All steps of the assay were completed within 50 min, its detection limit was 1 parasite/μL, and it was highly specific for P. falciparum. This effortless detection system with high sensitivity and specificity could provide an alternative choice for malaria diagnostics in resource-limited regions.

Selection of DNA Aptamers for Root Exudate l-Serine Using Multiple Selection Strategies

Agricultural biosensing can aid decisions about crop health and maintenance, because crops release root exudates that can inform about their status. l-Serine has been found to be indicative of nitrogen uptake in wheat and canola. The development of a biosensor for l-serine could allow farmers to monitor crop nutrient demands more precisely. The development of robust l-serine-binding DNA aptamers is described. Because small molecules can be challenging targets for Systematic Evolution of Ligands by EXponential enrichment (SELEX), three separate DNA libraries were used for SELEX experiments. A l-homocysteine aptamer was randomized to create a starting library for a l-serine selection (randomized SELEX). The final selection rounds of the l-homocysteine selection were also used as a starting library for l-serine (redirected SELEX). Finally, an original DNA library was used (original SELEX). All three SELEX experiments produced l-serine-binding aptamers with micromolar affinity, with Red.1 aptamer having a K_d of 7.9 ± 3.6 μM. Truncation improved the binding affinity to 5.2 ± 2.7 μM, and from this sequence, a Spiegelmer with improved nuclease resistance was created with a K_d of 2.0 ± 0.8 μM. This l-serine-binding Spiegelmer has the affinity and stability to be incorporated into aptamer-based biosensors for agricultural applications.

Recent advances in understanding oligonucleotide aptamers and their applications as therapeutic agents

The innovative discovery of aptamers was based on target-specific treatment in clinical diagnostics and therapeutics. Aptamers are synthetic, single-stranded oligonucleotides, simply described as chemical antibodies, which can bind to diverse targets with high specificity and affinity. Aptamers are synthesized by the SELEX technique, and possess distinctive properties as small size (10-50 kDa), higher stability, easy manufacture and less immunogenicity. These oligonucleotides are easily degraded by nucleases, so require some important modifications like capping and incorporation of modified nucleotides. RNA aptamers can be modified chemically on 2' positions using -NH3, -F, -deoxy, or -OMe groups to enhance their nuclease resistance. Aptamers have been employed for multiple purposes, as direct drugs or aptamer-drug conjugates targeted against different diseased cells. Different aptamer-conjugated nanovehicles (e.g., micelles, liposomes, silica nano-shells) have been designed to transport diverse anticancer-drugs like doxorubicin and cisplatin in bulk to minimize systemic cytotoxicity. Some drug-loaded nanovehicles (up to 97% loading capacity) and conjugated with specific aptamer resulted in more than 60% tumor inhibition as compared to unconjugated drug-loaded nanovehicles which showed only 31% cancer inhibition. In addition, aptamers have been widely used in basic research, food safety, environmental monitoring, clinical diagnostics and therapeutics. Different FDA-approved RNA and DNA aptamers are now available in the market, used for the treatment of diverse diseases, especially cancer. These aptamers include Macugen, Pegaptanib, etc. Despite a good progress in aptamer use, the present-day chemotherapeutics and drug targeting systems still face great challenges. Here in this review article, we are discussing nucleic acid aptamers, preparation, role in the transportation of different nanoparticle vehicles and their applications as therapeutic agents.

A label-free aptasensor for highly sensitive detection of homocysteine based on gold nanoparticles

In the present study, an original electrode fabrication approach was devised to create a label free sensitive electrochemical aptasensor for the detection of Homocysteine (Hcy) (Homocysteine signal was used for detection). To bind certain targets, synthetic oligonucleotides used as aptamers (APs) were specifically selected. Aptamers are substitutes for antibodies for analytical devices because of their sensitivity and high affinity. In this study, Hcy-Binding-Aptamer (HBA) was grafted onto the surface of Au nanoparticles/Glassy Carbon Electrode (Au/GCE) in order to create an aptasensor. The effects of buffer concentration, buffer type, interaction time, and aptamer concentration were investigated and optimized. In addition, Differential Pulse Voltammetry (DPV) was implemented to identify homocysteine. Favorable performance was achieved at a detection limit of 0.01 μM (S/N = 3) and linear range 0.05-20.0 μM. Furthermore, the fabricated aptasensor displayed desirable stability and reproducibility. The developed electrochemical aptasensor was found to have reasonable selectivity for the detection of homocysteine in the presence of cysteine and methionine. Analysis of real samples showed good ability of the proposed homocysteine biosensor to provide sensitive, quick, easy, and cost effective measurement of homocysteine in human blood serum and urine samples.

An aptamer-based colorimetric lateral flow assay for the detection of human epidermal growth factor receptor 2 (HER2)

An aptamer-based colorimetric lateral flow assay was developed for the detection of human epidermal growth factor receptor 2 (HER2). In this study, two approaches were examined using HER2 binding aptamers and gold nanoparticles. The first method used was a solution-based adsorption-desorption colorimetric approach wherein aptamers were adsorbed onto the gold nanoparticle surface. Upon the addition of HER2, HER2 binds specifically with its aptamer, releasing the gold nanoparticles. Addition of NaCl then induces the formation of gold nanoparticle aggregates. This leads to a color change from red to blue and a detection limit of 10 nM was achieved. The second method used an adsorption-desorption colorimetric lateral flow assay approach wherein biotin-modified aptamers were adsorbed onto the gold nanoparticle surface in the absence of HER2. In the presence of HER2, HER2 specifically binds with its aptamer leading to release of the gold nanoparticles. These solutions were applied to the lateral flow assay format and a detection limit of 20 nM was achieved. Both colorimetric and lateral flow assays are inexpensive, simple, rapid to perform and produce results visible to the naked-eye.

Cardiovascular therapies utilizing targeted delivery of nanomedicines and aptamers

Cardiovascular ailments are the foremost trigger of death in the world today, including myocardial infarction and ischemic heart diseases. To date, extraordinary measures have been prescribed, from the perspectives of both conventional medical therapies and surgeries, to enforce cardiac cell regeneration post cardiac traumas, albeit with limited long-term success. The prospects of successful heart transplants are also grim, considering exorbitant costs and unavailability of suitable donors in most cases. From the perspective of cardiac revascularization, use of nanoparticles and nanoparticle mediated targeted drug delivery have garnered substantial attention, attributing to both active and passive heart targeting, with enhanced target specificity and sensitivity. This review focuses on this aspect, while outlining the progress in targeted delivery of nanomedicines in the prognosis and subsequent therapy of cardiovascular disorders, and recapitulating the benefits and intrinsic challenges associated with the incorporation of nanoparticles. This article categorically provides an overview of nanoparticle-mediated targeted delivery systems and their implications in handling cardiovascular diseases, including their intrinsic benefits and encountered procedural trials and challenges. Additionally, the solicitations of aptamers in targeted drug delivery with identical objectives, are presented. This includes a detailed appraisal on various aptamer-navigated nanoparticle targeted delivery platforms in the diagnosis and treatment of cardiovascular maladies. Despite a few impending challenges, subject to additional investigations, both nanoparticles as well as aptamers show a high degree of promise, and pose as the next generation of drug delivery vehicles, in targeted cardiovascular therapy.

Comparison of turn-on and ratiometric fluorescent G-quadruplex aptasensor approaches for the detection of ATP

Two fluorescent aptasensor methods were developed for the detection of ATP in biochemical systems. The first method consisted of a label-free fluorescent "turn-on" approach using a guanine-rich ATP aptamer sequence and the DNA-binding agent berberine complex. In the presence of ATP, the ATP preferentially binds with its aptamer and conformationally changes into a G-quadruplex structure. The association of berberine with the G-quadruplex results in the enhancement of the fluorescence signal of the former. The detection limit of ATP was found to be 3.5 μM. Fluorescence, circular dichroism and melting temperature (T_m) experiments were carried out to confirm the binding specificity and structural changes. The second method employs the ratiometric fluorescent approach based on the Forster resonance energy transfer (FRET) for the detection of ATP using berberine along with a quencher (AuNRs, AgNPs) and a fluorophore (red quantum dots (RQDs), carbon dots (CDs)) labeled at 5' and 3' termini of the ATP-binding aptamer sequence. Upon addition of ATP and berberine, ATP specifically binds with its aptamer leading to the formation of G-quadruplex, and similarly, berberine also binds to the G-quadruplex. This leads to an enhancement of fluorescence of berberine while that of RQD and CDs were significantly quenched via FRET. The respective detection limits calculated were 3.6 μM and 3.8 μM, indicating these fluorescent aptasensor methods may be used for a wide variety of small molecules. Graphical abstract.

A surface plasmon field-enhanced fluorescence reversible split aptamer biosensor

Surface plasmon field-enhanced fluorescence is reported for the readout of a heterogeneous assay that utilizes low affinity split aptamer ligands. Weak affinity ligands that reversibly interact with target analytes hold potential for facile implementation in continuous monitoring biosensor systems. This functionality is not possible without the regeneration of more commonly used assays relying on high affinity ligands and end-point measurement. In fluorescence-based sensors, the use of low affinity ligands allows avoiding this step but it imposes a challenge associated with the weak optical response to the specific capture of the target analyte which is also often masked by a strong background. The coupling of fluorophore labels with a confined field of surface plasmons is reported for strong amplification of the fluorescence signal emitted from the sensor surface and its efficient discrimination from the background. This optical scheme is demonstrated for time-resolved analysis of chosen model analytes - adenoside and adenosine triphosphate - with a split aptamer that exhibits an equilibrium affinity binding constant between 0.73 and 1.35 mM. The developed biosensor enables rapid and specific discrimination of target analyte concentration changes from low μM to mM in buffer as well as in 10% serum.

Electrochemically prepared oxygen and sulfur co-doped graphitic carbon nitride quantum dots for fluorescence determination of copper and silver ions and biothiols

Although great advances have been achieved in synthesis of fluorescent graphitic carbon nitride quantum dots (g-C₃N₄-dots), it is still challenging to develop g-C₃N₄-dots with high fluorescence quantum yield (FLQY) and multiple sensing functionalities. Herein, the oxygen and sulfur co-doped graphitic carbon nitride quantum dots (OS-g-C₃N₄-dots) with high FLQY of 33.9% were firstly synthesized by a simple electrochemical "tailoring" process. It was found that OS-g-C₃N₄-dots could specifically bind copper ions (Cu²⁺) and silver ions (Ag⁺), accompanied with a dramatic "turn-off" fluorescence response. With the help of different masking agents, OS-g-C₃N₄-dots are able to selectively detect Cu²⁺ and Ag⁺. Furthermore, the generated OS-g-C₃N₄-dots/Ag⁺ displayed a "turn-on" fluorescent response specific to biothiols (HCy, Cys and GSH). Therefore, the multiple functional sensing platforms based on "ON-OFF-ON" fluorescence response of OS-g-C₃N₄-dots for the detection of Cu²⁺, Ag⁺ and biothiols were constructed. Under the optimal conditions, the detection limits of Cu²⁺, Ag⁺, HCy, Cys and GSH were as low as 7.0 × 10^-10 M, 2.0 × 10^-9 M, 1.0 × 10^-8 M, 1.0 × 10^-8 M and 8.4 × 10^-9 M, respectively. Moreover, the prepared platforms could be successfully applied to the determination of Cu²⁺, Ag⁺ and biothiols in practical samples and exhibited excellent sensitivity and selectivity.

Aptamers for DNA Damage and Repair

DNA is damaged on a daily basis, which can lead to heritable mutations and the activation of proto-oncogenes. Therefore, DNA damage and repair are critical risk factors in cancer, aging and disease, and are the underlying bases of most frontline cancer therapies. Much of our current understanding of the mechanisms that maintain DNA integrity has been obtained using antibody-based assays. The oligonucleotide equivalents of antibodies, known as aptamers, have emerged as potential molecular recognition rivals. Aptamers possess several ideal properties including chemical stability, in vitro selection and lack of batch-to-batch variability. These properties have motivated the incorporation of aptamers into a wide variety of analytical, diagnostic, research and therapeutic applications. However, their use in DNA repair studies and DNA damage therapies is surprisingly un-tapped. This review presents an overview of the progress in selecting and applying aptamers for DNA damage and repair research.

Comparison of In-Solution Biorecognition Properties of Aptamers against Ochratoxin A

Ochratoxin A (OTA) is a mycotoxin produced as a secondary metabolite by several species of Aspergillus and Penicillium and frequently found as a natural contaminant in a wide range of food commodities. Novel and robust biorecognition agents for detecting this molecule are required. Aptamers are artificial nucleic acid ligands able to bind with high affinity and specificity to a given target molecule. In the last few years, three separate research groups have selected aptamers for ochratoxin A. While each of these three families of aptamers have been incorporated into various methods for detecting OTA, it is unclear if each aptamer candidate is better suited for a particular application. Here, we perform the first head-to-head comparison of solution-based binding parameters for these groups of aptamers. Based on our results, we provide recommendations for the appropriate choice of aptamer for incorporation into solution-based biorecognition assays and applications.

Graphene oxide-based selection and identification of ofloxacin-specific single-stranded DNA aptamers

ssDNA aptamers specific to ofloxacin with high affinity were screened using graphene oxide-SELEX.

DNA Aptamers in the Diagnosis and Treatment of Human Diseases

Aptamers have a promising role in the field of life science and have been extensively researched for application as analytical tools, therapeutic agents and as vehicles for targeted drug delivery. Compared with RNA aptamers, DNA aptamers have inherent advantages in stability and facility of generation and synthesis. To better understand the specific potential of DNA aptamers, an overview of the progress in the generation and application of DNA aptamers in human disease diagnosis and therapy are presented in this review. Special attention is given to researches that are relatively close to practical application. DNA aptamers are expected to have great potential in the diagnosis and treatment of human diseases.

Comprehensive analytical comparison of strategies used for small molecule aptamer evaluation

Nucleic acid aptamers are versatile molecular recognition agents that bind to their targets with high selectivity and affinity. The past few years have seen a dramatic increase in aptamer development and interest for diagnostic and therapeutic applications. As the applications for aptamers expand, the need for a more standardized, stringent, and informative characterization and validation methodology increases. Here we performed a comprehensive analysis of a panel of conventional affinity binding assays using a suite of aptamers for the small molecule target ochratoxin A (OTA). Our results highlight inconsistency between conventional affinity assays and the need for multiple characterization strategies. To mitigate some of the challenges revealed in our head-to-head comparison of aptamer binding assays, we further developed and evaluated a set of novel strategies that facilitate efficient screening and characterization of aptamers in solution. Finally, we provide a workflow that permits rapid and robust screening, characterization, and functional verification of aptamers thus improving their development and integration into novel applications.

Selection and characterization of a novel DNA aptamer for label-free fluorescence biosensing of ochratoxin A

Nucleic acid aptamers are emerging as useful molecular recognition tools for food safety monitoring. However, practical and technical challenges limit the number and diversity of available aptamer probes that can be incorporated into novel sensing schemes. This work describes the selection of novel DNA aptamers that bind to the important food contaminant ochratoxin A (OTA). Following 15 rounds of in vitro selection, sequences were analyzed for OTA binding. Two of the isolated aptamers demonstrated high affinity binding and selectivity to this mycotoxin compared to similar food adulterants. These sequences, as well as a truncated aptamer (minimal sequence required for binding), were incorporated into a SYBR® Green I fluorescence-based OTA biosensing scheme. This label-free detection platform is capable of rapid, selective, and sensitive OTA quantification with a limit of detection of 9 nM and linear quantification up to 100 nM.

Smart materials based on DNA aptamers: taking aptasensing to the next level

"Smart" materials are an emerging category of multifunctional materials with physical or chemical properties that can be controllably altered in response to an external stimulus. By combining the standard properties of the advanced material with the unique ability to recognize and adapt in response to a change in their environment, these materials are finding applications in areas such as sensing and drug delivery. While the majority of these materials are responsive to physical or chemical changes, a particularly exciting area of research seeks to develop smart materials that are sensitive to specific molecular or biomolecular stimuli. These systems require the integration of a molecular recognition probe specific to the target molecule of interest. The ease of synthesis and labeling, low cost, and stability of DNA aptamers make them uniquely suited to effectively serve as molecular recognition probes in novel smart material systems. This review will highlight current work in the area of aptamer-based smart materials and prospects for their future applications.