Toward Integrated Molecular Lateral Flow Diagnostic Tests Using Advanced Micro- and Nanotechnology

Point-of-Care Testing: The Convergence of Innovation and Accessibility in Diagnostics

Over the years, the evolution of point-of-care testing (POCT) has been driven by technological advancements in materials, design, and artificial intelligence, as well as breakthrough developments in wearable technologies. These innovations are shifting diagnostics from centralized medical facilities to individual homes, meeting the growing demand for personalized healthcare. This Review explores recent advancements in binding-based assay technologies over the past two years, focusing on platforms such as traditional flow assays (lateral and vertical flow), fully integrated microfluidic devices, and wearable biosensor-integrated systems for POCT applications. It emphasizes the role of optical and electrochemical detection methods, which are essential for ensuring the sensitivity, specificity, and reliability required in a POCT. POCT technologies offer advantages including ease of use, high diagnostic accuracy, rapid clinical assessment, and cost-effectiveness in manufacturing and consumables. Additionally, the Review highlights current challenges and future perspectives for delivering personalized healthcare through portable and wearable POCT systems that operate on a sample-in, result-out basis.

Dual-Function SERS Microdroplet Sensor for Rapid Differentiation of Influenza a and SARS-CoV-2

This study presents the development of a dual-function microdroplet sensor utilizing surface-enhanced Raman scattering (SERS) technology to identify and quantify Influenza A and COVID-19 viruses. The proposed microfluidic device incorporates compartments for two-phase segmented droplet generation, merging, splitting, and detection. Both viral strains were identified by isolating magnetic antibody-antigen complexes from the liquid medium using a magnetized bar embedded in the microfluidic channel. Concurrent Raman spectroscopic readings were obtained as suspended droplets containing residual SERS-active nanoparticles traversed the interrogation zone of the focused laser beam. Precise quantitative analysis was accomplished by correcting characteristic Raman peak intensities for both viruses with internal standards, while ensemble averaging Raman signals from multiple droplets ensured high reproducibility. This dual-function SERS microdroplet sensor represents a novel in vitro diagnostic approach capable of rapidly distinguishing between COVID-19 and Influenza A with high sensitivity and reproducibility. When coupled with a portable Raman spectrophotometer, the device shows significant potential as a diagnostic tool for swift and in situ detection of both viral pathogens.

Quantum dots for biosensing: Classification and applications

Quantum dots (QDs) are the smallest nanomaterials (2-10 nm), with unique optical and electronic properties. Thanks to these properties, QDs have been standing during the last years as signal tags for different applications, including bioimaging, fluorescent biosensors and electrochemical assays. In this review, we explore the current state-of-the art on these nanomaterials, differentiating them between semiconductor and carbon-based QDs. Also, the review focuses on their unique advantages as transducers in different biosensing platforms.

A Fully Integrated ICP-Enriched and Nanozyme-Catalyzed Lateral Flow Assay for cfDNA Detection in Whole Blood

Rapid and sensitive detection of Epstein-Barr virus cell-free DNA (EBV cfDNA) is crucial for early diagnosis and monitoring of nasopharyngeal carcinoma (NPC), but accessibility to screening is limited by complicated and costly conventional DNA isolation and purification approaches. Here, a fully integrated ion concentration polarization (ICP)-enriched and nanozyme-catalyzed lateral flow assay (ICP-cLFA) is developed, enabling total analysis of EBV cfDNA in whole blood samples, with DNA isolation, pre-concentration, and amplification performed on a microfluidic chip, consequently providing the signal readout within 75 min. Specifically, ICP preconcentration and amplification steps, together with target recognition catalyzed by a platinum-decorated mesoporous gold nanosphere (MGNS@Pt) nanozyme, result in an ultralow detection limit of 4 aM in standard cfDNA samples and 100 aM in whole blood from NPC-bearing rats. The high sensitivity and specificity of the ICP-cLFA suggest strong potential for cfDNA screening in resource-limited clinical and field applications.

Plasmonic nanoparticle sensors: current progress, challenges, and future prospects

Plasmonic nanoparticles (NPs) have played a significant role in the evolution of modern nanoscience and nanotechnology in terms of colloidal synthesis, general understanding of nanocrystal growth mechanisms, and their impact in a wide range of applications. They exhibit strong visible colors due to localized surface plasmon resonance (LSPR) that depends on their size, shape, composition, and the surrounding dielectric environment. Under resonant excitation, the LSPR of plasmonic NPs leads to a strong field enhancement near their surfaces and thus enhances various light-matter interactions. These unique optical properties of plasmonic NPs have been used to design chemical and biological sensors. Over the last few decades, colloidal plasmonic NPs have been greatly exploited in sensing applications through LSPR shifts (colorimetry), surface-enhanced Raman scattering, surface-enhanced fluorescence, and chiroptical activity. Although colloidal plasmonic NPs have emerged at the forefront of nanobiosensors, there are still several important challenges to be addressed for the realization of plasmonic NP-based sensor kits for routine use in daily life. In this comprehensive review, researchers of different disciplines (colloidal and analytical chemistry, biology, physics, and medicine) have joined together to summarize the past, present, and future of plasmonic NP-based sensors in terms of different sensing platforms, understanding of the sensing mechanisms, different chemical and biological analytes, and the expected future technologies. This review is expected to guide the researchers currently working in this field and inspire future generations of scientists to join this compelling research field and its branches.

Surface Functionalization of Gold Nanoparticles Using Alkyne Derivatives: Applications in Chemical Sensing

Colloidal gold nanoparticles (AuNPs) are important nanomaterials for chemical sensing and therapeutics. For their application, it is vital to develop a reliable and robust surface functionalization method that can be applied to diverse functional molecules and offer better stability under harsh biological conditions. Currently, thiol (SH) is the most commonly used functional group for forming stable covalent bonds with AuNPs. However, thiolated molecules typically require complicated preparation procedures, are susceptible to oxidation, and are not compatible with many electrophiles and reducing groups. In this study, we report that surface functionalization of AuNPs can be achieved using alkyne derivatives, which exhibit several advantages over classical thiolation and peptide-bond methods, including straightforward preparation of alkyne derivatives, rapid and simple conjugation in buffers and complex media, higher conjugation efficiency, long-term stability, and resistance to decomposition under harsh conditions. Several alkynylated biotin and fluorescein derivatives are prepared, and the alkynylated-AuNPs are characterized using a lateral flow assay, gel electrophoresis, and spectroscopy techniques to investigate the conjugation efficiencies, size distributions, protein interaction properties, and binding mode of the Au-alkyne bond. We also demonstrate that alkynylated-AuNPs can be used for the sensitive detection of hydrogen peroxide and streptavidin proteins.

Lateral flow immunoassay for simultaneous detection of C. difficile, MRSA, and K. pneumoniae

Mainly performed within a rapid diagnostic tests company, a lateral flow (LF) system using gold nanoparticles (AuNPs) as transducers is presented able to detect three bacteria of interest, of relevance for antimicrobial resistance (AMR): Clostridioides difficile, methicillin-resistant Staphylococcus aureus (MRSA), and Klebsiella pneumoniae, with a limit of detection of 25 ng/mL of glutamate dehydrogenase (GDH) for C. difficile, 36 ng/mL of penicillin-binding protein 2a (PBP2a) for MRSA, and 4 × 106 CFU/mL for K. pneumoniae. The system showed good results with bacteria culture samples, is user-friendly, and suitable for rapid testing, as the results are obtained within 15 min.

Improved sensitivity in paper-based microfluidic analytical devices using a pH-responsive valve for nitrate analysis

This paper presents an innovative application of chitosan material to be used as pH-responsive valves for the precise control of lateral flow in microfluidic paper-based analytical devices (μPADs). The fabrication of μPADs involved wax printing, while pH-responsive valves were created using a solution of chitosan in acetic acid. The valve-forming solution was applied, and ready when dry; by exposure to acidic solutions, the valve opens. Remarkably, the valves exhibited excellent compatibility with alkaline, neutral, and acidic solutions with a pH higher than 4. The valve opening process had no impact on the flow rate and colorimetric analysis. The potential of chitosan valves used for flow control was demonstrated for μPADs employed for nitrate determination. Valves were used to increase the conversion time of nitrate to nitrite, which was further analyzed using the Griess reaction. The μPAD showed a linear response in the concentration range of 10-100 μmol L-1, with a detection limit of 5.4 μmol L-1. As a proof of concept, the assay was successfully applied to detect nitrate levels in water samples from artificial lakes of recreational parks. For analyses that require controlled kinetics and involve multiple sequential steps, the use of chitosan pH-responsive valves in μPADs is extremely valuable. This breakthrough holds great potential for the development of simple and high-impact microfluidic platforms that can cater to a wide range of analytical chemistry applications.

The Evaluation of Clinical Applications for the Detection of the Alzheimer's Disease Biomarker GFAP

One of the most prevalent neurodegenerative diseases is Alzheimer's disease (AD). The hallmarks of AD include the accumulation of amyloid plaques and neurofibrillary tangles, which cause related secondary diseases, progressive neurodegeneration, and ultimately death. The most prevalent cell type in the human central nervous system, astrocytes, are crucial for controlling neuronal function. Glial fibrillary acidic protein (GFAP) is released from tissue into the bloodstream due to astrocyte breakdown in neurological diseases. Increased levels of GFAP in the serum can function as blood markers and be an effective prognostic indicator to help diagnose neurological conditions early on, from stroke to neurodegenerative diseases. The human central nervous system (CNS) is greatly affected by diseases associated with blood GFAP levels. These include multiple sclerosis, intracerebral hemorrhage, glioblastoma multiforme, traumatic brain injuries, and neuromyelitis optica. GFAP demonstrates a strong diagnostic capacity for projecting outcomes following an injury. Furthermore, the increased ability to identify GFAP protein fragments helps facilitate treatment, as it allows continuous screening of CNS injuries and early identification of potential recurrences. GFAP has recently gained attention due to data showing that the plasma biomarker is effective in identifying AD pathology. AD accounts for 60-70% of the approximately 50 million people with dementia worldwide. It is critical to develop molecular markers for AD, whose number is expected to increase to about 3 times and affect humans by 2050, and to investigate possible targets to confirm their effectiveness in the early diagnosis of AD. In addition, most diagnostic methods currently used are image-based and do not detect early disease, i.e. before symptoms appear; thus, treatment options and outcomes are limited. Therefore, recently developed methods such as point-of-care (POC), on-site applications, and enzyme-linked immunosorbent assay-polymerase chain reaction (ELISA-PCR) that provide both faster and more accurate results are gaining importance. This systematic review summarizes published studies with different approaches such as immunosensor, lateral flow, POC, ELISA-PCR, and molecularly imprinted polymer using GFAP, a potential blood biomarker to detect neurological disorders. Here, we also provide an overview of current approaches, analysis methods, and different future detection strategies for GFAP, the most popular biosensing field.

Colorimetric sensing for translational applications: from colorants to mechanisms

Colorimetric sensing offers instant reporting via visible signals. Versus labor-intensive and instrument-dependent detection methods, colorimetric sensors present advantages including short acquisition time, high throughput screening, low cost, portability, and a user-friendly approach. These advantages have driven substantial growth in colorimetric sensors, particularly in point-of-care (POC) diagnostics. Rapid progress in nanotechnology, materials science, microfluidics technology, biomarker discovery, digital technology, and signal pattern analysis has led to a variety of colorimetric reagents and detection mechanisms, which are fundamental to advance colorimetric sensing applications. This review first summarizes the basic components (e.g., color reagents, recognition interactions, and sampling procedures) in the design of a colorimetric sensing system. It then presents the rationale design and typical examples of POC devices, e.g., lateral flow devices, microfluidic paper-based analytical devices, and wearable sensing devices. Two highlighted colorimetric formats are discussed: combinational and activatable systems based on the sensor-array and lock-and-key mechanisms, respectively. Case discussions in colorimetric assays are organized by the analyte identities. Finally, the review presents challenges and perspectives for the design and development of colorimetric detection schemes as well as applications. The goal of this review is to provide a foundational resource for developing colorimetric systems and underscoring the colorants and mechanisms that facilitate the continuing evolution of POC sensors.

"Three-in-One" Plasmonic Au@PtOs Nanocluster Driven Lateral Flow Assay for Multimodal Cancer Exosome Biosensing

Exploiting the multiple properties of nanozymes for the multimode lateral flow assay (LFA) is urgently required to improve the accuracy and versatility. Herein, we developed a novel plasmonic Au nanostar@PtOs nanocluster (Au@PtOs) as a multimode signal tag for LFA detection. Based on the PtOs bimetallic nanocluster doping strategy, Au@PtOs can indicate both excellent SERS enhancement and nanozyme catalytic activity. Meanwhile, Au@PtOs displays a better photothermal effect than that of Au nanostars. Therefore, catalytic colorimetric/SERS/temperature three-mode signals can be read out based on the Au@PtOs nanocomposite. The Au@PtOs was combined with LFA and applied for breast cancer exosome detection. The detection limit for the colorimetric/SERS/temperature mode was 2.6 × 103/4.1 × 101/4.6 × 102 exosomes/μL, respectively, which was much superior to the common Au nanoparticles LFA (∼105 exosomes/μL). Moreover, based on the fingerprint molecular recognition ability of the SERS mode, exosome phenotypes derived from different breast cancer cell lines can be discriminated easily. Based on the convenient visual colorimetric mode and sensitive SERS/temperature quantitative modes, Au@PtOs driven LFA can satisfy the requirements of accurate and flexible multimodal sensing in different application scenarios.

Unleashing inkjet-printed nanostructured electrodes and battery-free potentiostat for the DNA-based multiplexed detection of SARS-CoV-2 genes

Following the global COVID-19 pandemic triggered by SARS-CoV-2, the need for rapid, specific and cost-effective point-of-care diagnostic solutions remains paramount. Even though COVID-19 is no longer a public health emergency, the disease still poses a global threat leading to deaths, and it continues to change with the risk of new variants emerging causing a new surge in cases and deaths. Here, we address the urgent need for rapid, cost-effective and point-of-care diagnostic solutions for SARS-CoV-2. We propose a multiplexed DNA-based sensing platform that utilizes inkjet-printed nanostructured gold electrodes and an inkjet-printed battery-free near-field communication (NFC) potentiostat for the simultaneous quantitative detection of two SARS-CoV-2 genes, the ORF1ab and the N gene. The detection strategy based on the formation of an RNA-DNA sandwich structure leads to a highly specific electrochemical output. The inkjet-printed nanostructured gold electrodes providing a large surface area enable efficient binding and increase the sensitivity. The inkjet-printed battery-free NFC potentiostat enables rapid measurements and real-time data analysis via a smartphone application, making the platform accessible and portable. With the advantages of speed (5 min), simplicity, sensitivity (low pM range, ∼450% signal gain) and cost-effectiveness, the proposed platform is a promising alternative for point-of-care diagnostics and high-throughput analysis that complements the COVID-19 diagnostic toolkit.

AI-Enhanced Visual-Spectral Synergy for Fast and Ultrasensitive Biodetection of Breast Cancer-Related miRNAs

In biomedical testing, artificial intelligence (AI)-enhanced analysis has gradually been applied to the diagnosis of certain diseases. This research employs AI algorithms to refine the precision of integrative detection, encompassing both visual results and fluorescence spectra from lateral flow assays (LFAs), which signal the presence of cancer-linked miRNAs. Specifically, the color shift of gold nanoparticles (GNPs) is paired with the red fluorescence from nitrogen vacancy color centers (NV-centers) in fluorescent nanodiamonds (FNDs) and is integrated into LFA strips. While GNPs amplify the fluorescence of FNDs, in turn, FNDs enhance the color intensity of GNPs. This reciprocal intensification of fluorescence and color can be synergistically augmented with AI algorithms, thereby improving the detection sensitivity for early diagnosis. Supported by the detection platform based on this strategy, the fastest detection results with a limit of detection (LOD) at the fM level and the R2 value of ∼0.9916 for miRNA can be obtained within 5 min. Meanwhile, by labeling the capture probes for miRNA-21 and miRNA-96 (both of which are early indicators of breast cancer) on separate T-lines, simultaneous detection of them can be achieved. The miRNA detection methods employed in this study may potentially be applied in the future for the early detection of breast cancer.

Affinity-Switchable Interaction of Biotin and Streptavidin for the Signal-ON Detection of Small Molecules

Lateral flow assay (LFA) based on gold nanoparticles (AuNPs) is a widely used analytical device for the rapid analysis of environmental hazards and biomarkers. Typically, a sandwich-type format is used for macromolecule detection, in which the appearance of a red test line indicates a positive result (Signal-ON). In contrast, small molecule detection usually relies on a competitive assay, where the absence of a test line indicates positive testing (Signal-OFF). However, such a "Signal-OFF" reading is usually detected within a narrower dynamic range and tends to generate false-negative signals at a low concentration. Moreover, inconsistent readings between macromolecule and small molecule testing might lead to misinterpretation when used by nonskilled individuals. Herein, we report a "Signal-ON" small molecule competitive assay based on the sterically modulated affinity-switchable interaction of biotin and streptavidin. In the absence of a small molecule target, a large steric hindrance can be imposed on the biotin to prevent interaction with streptavidin. However, in the presence of the small molecule target, this steric effect is removed, allowing the biotin to bind to streptavidin and generate the desired test line. In this article, we demonstrate the selective detection of two small molecule drugs, sulfonamides and trimethoprim, using this simple and modular affinity-switchable lateral flow assay (ASLFA). We believe that this affinity-switchable approach can also be adapted in drug discovery and clinical diagnosis, where the competitive assay format is always used for the rapid analysis of small molecules.

Decorated DNA-Based Scaffolds as Lateral Flow Biosensors

Here we develop Lateral Flow Assays (LFAs) that employ as functional elements DNA-based structures decorated with reporter tags and recognition elements. We have rationally re-engineered tile-based DNA tubular structures that can act as scaffolds and can be decorated with recognition elements of different nature (i.e. antigens, aptamers or proteins) and with orthogonal fluorescent dyes. As a proof-of-principle we have developed sandwich and competitive multiplex lateral flow platforms for the detection of several targets, ranging from small molecules (digoxigenin, Dig and dinitrophenol, DNP), to antibodies (Anti-Dig, Anti-DNP and Anti-MUC1/EGFR bispecific antibodies) and proteins (thrombin). Coupling the advantages of functional DNA-based scaffolds together with the simplicity of LFAs, our approach offers the opportunity to detect a wide range of targets with nanomolar sensitivity and high specificity.

Insulin detection in diabetes mellitus: challenges and new prospects

Tremendous progress has been made towards achieving tight glycaemic control in individuals with diabetes mellitus through the use of frequent or continuous glucose measurements. However, in patients who require insulin, accurate dosing must consider multiple factors that affect insulin sensitivity and modulate insulin bolus needs. Accordingly, an urgent need exists for frequent and real-time insulin measurements to closely track the dynamic blood concentration of insulin during insulin therapy and guide optimal insulin dosing. Nevertheless, traditional centralized insulin testing cannot offer timely measurements, which are essential to achieving this goal. This Perspective discusses the advances and challenges in moving insulin assays from traditional laboratory-based assays to frequent and continuous measurements in decentralized (point-of-care and home) settings. Technologies that hold promise for insulin testing using disposable test strips, mobile systems and wearable real-time insulin-sensing devices are discussed. We also consider future prospects for continuous insulin monitoring and for fully integrated multisensor-guided closed-loop artificial pancreas systems.

The application of nanoparticles in point-of-care testing (POCT) immunoassays

The Covid-19 pandemic has led to greater recognition of the importance of the fast and timely detection of pathogens. Recent advances in point-of-care testing (POCT) technology have shown promising results for rapid diagnosis. Immunoassays are among the most extensive POCT assays, in which specific labels are used to indicate and amplify the immune signal. Nanoparticles (NPs) are above the rest because of their versatile properties. Much work has been devoted to NPs to find more efficient immunoassays. Herein, we comprehensively describe NP-based immunoassays with a focus on particle species and their specific applications. This review describes immunoassays along with key concepts surrounding their preparation and bioconjugation to show their defining role in immunosensors. The specific mechanisms, microfluidic immunoassays, electrochemical immunoassays (ELCAs), immunochromatographic assays (ICAs), enzyme-linked immunosorbent assays (ELISA), and microarrays are covered herein. For each mechanism, a working explanation of the appropriate background theory and formalism is articulated before examining the biosensing and related point-of-care (POC) utility. Given their maturity, some specific applications using different nanomaterials are discussed in more detail. Finally, we outline future challenges and perspectives to give a brief guideline for the development of appropriate platforms.

Biosensing strategies for the detection of SARS-CoV-2 nucleic acids

The COVID-19 pandemic had devastating effects throughout the world, producing a severe crisis in the health systems and in the economy of a long list of countries, even developed ones. Therefore, highly sensitive and selective analytical bioplatforms that allow the descentralized and fast detection of the severe acute respiratory síndrome coronavirus 2 (SARS-CoV-2), are extremely necessary. Since 2020, several reviews have been published, most of them focused on the different strategies to detect the SARS-CoV-2, either from RNA, viral proteins or host antibodies produced due to the presence of the virus. In this review, the most relevant biosensors for the detection of SARS-CoV-2 RNA are particularly addressed, with special emphasis on the discussion of the biorecognition layers and the different schemes for transducing the hybridization event.

Comparison of Biosensing Methods Based on Different Isothermal Amplification Strategies: A Case Study with Erwinia amylovora

Isothermal amplifications allow for the highly sensitive detection of nucleic acids, bypassing the use of instrumental thermal cycling. This work aimed to carry out an experimental comparison of the four most promising techniques: recombinase polymerase amplification (RPA) and loop-mediated isothermal amplification (LAMP) coupled with lateral flow test or coupled with additional amplification based on CRISPR/Cas12a resulting from the fluorescence of the Cas12a-cleaved probe. To compare the four amplification techniques, we chose the bacterial phytopathogen Erwinia amylovora (causative agent of fire blight), which has a quarantine significance in many countries and possesses a serious threat to agriculture. Three genes were chosen as the targets and primers were selected for each one (two for RPA and six for LAMP). They were functionalized by labels (biotin, fluorescein) at the 5' ends for amplicons recognition by LFT. As a result, we developed LAMP-LFT, LAMP-CRISPR/Cas, RPA-LFT, and RPA-CRISPR/Cas for E. amylovora detection. The detection limit was 10⁴ CFU/mL for LAMP-LFT, 10³ CFU/mL for LAMP-CRISPR/Cas, and 10² CFU/mL for RPA-LFT and RPA-CRISPR/Cas. The results of four developed test systems were verified by qPCR on a panel of real samples. The developed assays based on RPA, LAMP, CRISPR/Cas12a, and LFT are rapid (30-55 min), user-friendly, and highly sensitive for E. amylovora detection. All proposed detection methods can be applied to fire blight diagnosis and effective management of this disease.