Triple signal amplification of graphene film, polybead carried gold nanoparticles as tracing tag and silver deposition for ultrasensitive electrochemical immunosensing

A triple signal amplification strategy was designed for ultrasensitive immunosensing of cancer biomarker. This strategy was achieved using graphene to modify immunosensor surface for accelerating electron transfer, poly(styrene-co-acrylic acid) microbead (PSA) carried gold nanoparticles (AuNPs) as tracing tag to label signal antibody (Ab(2)) and AuNPs induced silver deposition for anodic stripping analysis. The immunosensor was constructed by covalently immobilizing capture antibody on chitosan/electrochemically reduced graphene oxide film modified glass carbon electrode. The in situ synthesis of AuNPs led to the loading of numerous AuNPs on PSA surface and convenient labeling of the tag to Ab(2). With a sandwich-type immunoreaction, the AuNPs/PSA labeled Ab(2) was captured on the surface of an immunosensor to further induce a silver deposition process. The electrochemical stripping signal of the deposited silver nanoparticles in KCl was used to monitor the immunoreaction. The triple signal amplification greatly enhanced the sensitivity for biomarker detection. The proposed method could detect carcinoembryonic antigen with a linear range of 0.5 pg mL(-1) to 0.5 ng mL(-1) and a detection limit down to 0.12 pg mL(-1). The immunosensor exhibited good stability and acceptable reproducibility and accuracy, indicating potential applications in clinical diagnostics.     

Graphene oxide sheet-mediated silver enhancement for application to electrochemical biosensors

Functionalized graphene oxide (GO) sheets coupled with a signal amplification method based on the nanomaterial-promoted reduction of silver ions for the sensitive and selective detection of bacteria. This paper aims to develop an electrochemical route combined with GO sheet-mediated Ag enhancement for biological/chemical analyte detection. A linear relationship between the stripping response and the logarithm of the bacterial concentration was obtained using an electrochemical technique for concentrations ranging from 1.8 × 10(2) to 1.8 × 10(8) cfu mL(-1), with a slope of 15.28 and a correlation coefficient of 0.995. Dot blot assay was used as a conventional immunoassay method for comparison with the electrochemical method, as well as to observe the quality of the anti-sulfate-reducing bacteria (SRB) antibody (Ab) used in the immunosensor. The GO sheet-mediated silver enhancement holds great potential for the rapid analysis of protein, DNA, and pathogens.     

Polymer-functionalized silica nanosphere labels for ultrasensitive detection of tumor necrosis factor-alpha

A signal amplification strategy for sensitive detection of tumor necrosis factor-alpha (TNF-α) using quantum dots (QDs)-polymer-functionalized silica nanosphere as the label was proposed. In this approach, silica nanospheres with good monodispersity and uniform structure were employed as carriers for surface-initiated atom transfer radical polymerization of glycidyl methacrylate, which is readily available functional monomer that possessing easily transformable epoxy groups for subsequent CdTe QDs binding through ring-open reaction. Then, human anti rabbit TNF-α antibody (anti-TNF-α, Ab2, served as a model protein) was bonded to CdTe QDs-modified silica nanospheres coated with polymer to obtain QDs-polymer-functionalized silica nanosphere labels (Si/PGMA/QD/Ab2). The Si/PGMA/QD/Ab2 labels were attached onto a gold electrode surface through a subsequent "sandwich" immunoreaction. This reaction was confirmed by scanning electron microscopy (SEM) and fluorescence microscopic images. Enhanced sensitivity could be achieved by an increase of CdTe QD loading per immunoassay event, because of a large number of surface functional epoxy groups offered by the PGMA. As a result, the electrochemiluminescence (ECL) and square-wave voltammetry (SWV) measurements showed 10.0- and 5.5-fold increases in detection signals, respectively, in comparison with the unamplified method. The detection limits of 7.0 pg mL(-1) and 3.0 pg mL(-1) for TNF-α antibodies by ECL and SWV measurements, respectively, were achieved. The proposed strategy successfully demonstrated a simple, reproducible, specific, and potent method that can be expanded to detect other proteins and DNA.     

Magneto-controlled graphene immunosensing platform for simultaneous multiplexed electrochemical immunoassay using distinguishable signal tags

A novel flow-through multiplexed immunoassay protocol for simultaneous electrochemical determination of carcinoembryonic (CEA) and alpha-fetoprotein (AFP) in biological fluids was designed using biofunctionalized magnetic graphene nanosheets (MGO) as immunosensing probes and multifunctional nanogold hollow microspheres (GHS) as distinguishable signal tags. The probes were fabricated by means of co-immobilization of primary anti-CEA (Ab(1)) and anti-AFP (Ab(2)) antibodies on the Fe(3)O(4) nanoparticle-coated graphene nanosheets (MGO-Ab(1,2)). The reverse-micelle method was used for the synthesis of distinguishable signal tags by encapsulation of horseradish peroxide (HRP)-thionine and HRP-ferrocene into nanogold hollow microspheres, respectively, which were utilized as labels of the corresponding GHS-Ab(1) and GHS-Ab(2). A sandwich-type immunoassay format was employed for the online detection of CEA and AFP by coupling a flow-through detection cell with an external magnet. The assay was based on the catalytic reduction of H(2)O(2) at the various peak potentials in the presence of the corresponding mediators. Experimental results revealed that the multiplexed electrochemical immunoassay enabled the simultaneous monitoring of AFP and CEA in a single run with wide working ranges of 0.01-200 ng mL(-1) for AFP and 0.01-80 ng mL(-1) for CEA. The detection limits (LODs) for both analytes at 1.0 pg mL(-1) (at 3s(B)) were very low. No obvious nonspecific adsorption and cross-talk were observed during a series of analyses to detect target analytes. Intraassay and interassay coefficients of variation were <10%. Importantly, the methodology was evaluated for the analysis of clinical serum specimens, receiving a good correlation between the flow-through multiplexed electrochemical immunoassay and an electrochemiluminescence method as a reference.     

Versatile apoferritin nanoparticle labels for assay of protein

A versatile bioassay label based on marker-loaded apoferritin nanoparticles (MLANs) has been developed for sensitive protein detection. Dissociation and reconstitution characteristics at different pH as well as the special cavity structure of apoferritin provides a facile route to prepare nanoparticle labels and avoid the complicated and tedious synthesis process of conventional nanoparticle labels. The optical and electrochemical characteristics of the prepared nanoparticle labels are easily controlled by loading different optical or electrochemical markers. A fluorescence marker (fluorescein anion) and a redox marker [hexacyanoferrate(III)] were used as model markers to load into the cavity of apoferritin nanoparticles for microscopic fluorescence immunoassay and electrochemical immunoassay, respectively. Detection limits of 0.06 (0.39 pM) and 0.08 ng mL(-1) (0.52 pM) IgG were obtained with fluorescein MLAN and hexacyanoferrate MLANs, respectively. The new nanoparticle labels hold great promise for multiplex protein detection (in connection with nanoparticles loaded with different markers) and for enhancing the sensitivity of other bioassays.     

Quantum dot-based near-infrared electrochemiluminescent immunosensor with gold nanoparticle-graphene nanosheet hybrids and silica nanospheres double-assisted signal amplification

Near-infrared electrochemiluminescence (NIR ECL) from quantum dots (QDs) has aroused particular attention. However, whether it is possible to achieve NIR ECL sensing has remained an open question. In this article, we reported a NIR ECL immunosensor with amplification techniques for ultrasensitive and selective determination of biomarker. In this sensing platform, NIR-emitting CdTe/CdS core(small)/shell(thick) QDs were first selected as NIR ECL emitters. The NIR ECL nanoprobe (SiO(2)-QD-Ab2) was designed by covalent assembly of goat antihuman IgG antibody (Ab2) on CdTe/CdS QDs tagged silica nanospheres. Gold nanoparticle-graphene nanosheet (Au-GN) hybrids were prepared by a sonication-induced self-assembly and served as an effective matrix for initial antibodies (Ab1) attachment. After a sandwich immunoreaction, the functionalized silica nanosphere labels were captured onto the glass carbon electrode surface. Integrating the dual amplification from the promoting electron transfer rate of Au-GN hybrids and the increasing QD loading of SiO(2)-QD-Ab2 labels, the NIR ECL response from CdTe/CdS QDs enhanced 16.8-fold compared to the unamplified protocol and successfully fulfilled the ultrasensitive detection of human IgG (HIgG) with a detection limit of 87 fg mL(-1). Moreover, as a practical application, the proposed immunosensor was used to monitor HIgG level in human serum with satisfactory results obtained.     

Alkaline phosphatase-catalyzed silver deposition for electrochemical detection

Alkaline phosphatase (AP) is one of the most used enzymatic labels for the development of ELISAs, immunosensors, DNA hybridization assays, etc. This enzyme catalyzes the dephosphorylation of a substrate into a detectable product usually quantified by optical or electrochemical measurements. This work is based on a substrate (3-indoxyl phosphate) that produces a compound able to reduce silver ions in solution into a metallic deposit, which is localized where the enzymatic label AP is attached. The deposited silver is electrochemically stripped into solution and measured by anodic stripping voltammetry. Its application to an enzymatic genosensor on streptavidin-modified screen-printed carbon electrodes for the detection of virulence nucleic acid determinants of autolysin gene, exclusively present on the genome of the human pathogen Streptococcus pneumoniae, is described. Compared with the direct voltammetric detection of indigo carmine, the anodic stripping voltammetry of silver ions is 14-fold more sensitive.     

Bioassembled nanocircuits of Mo6S9-xIx nanowires for electrochemical immunodetection of estrone Hapten

We demonstrate a novel and highly sensitive electrochemical detection of estrone based on an immunosensor platform, composed of bioassembled nanocircuits of Mo 6S 9- x I x nanowires (MoSI NWs) covalently connected to anti-estrone antibodies. The one-step, label-free, and quantitative detection of estrone is realized by employing the [Ru(NH 3) 6] (3+/2+) redox ions to sense anti-estrone antibody and estrone interactions. The MoSI NWs/anti-estrone antibody nanocircuit architectures provide an amplification and conductive pathway for the specific electrochemical sensing of estrone hapten. A detection limit of 1.4 pg x mL (-1) was achieved in contrast to previous electrochemical techniques in which the sensitivity was limited to the nanomolar range.     

Disposable electrochemical immunosensor diagnosis device based on nanoparticle probe and immunochromatographic strip

We describe a disposable electrochemical immunosensor diagnosis device that integrates the immunochromatographic strip technique with an electrochemical immunoassay and exploits quantum dot (QD, CdS@ZnS) as labels for amplifying signal output. The device takes advantage of the speed and low cost of the conventional immunochromatographic strip test and the high sensitivity of the nanoparticle-based electrochemical immunoassay. A sandwich immunoreaction was performed on the immunochromatographic strip, and the captured QD labels in the test zone were determined by highly sensitive stripping voltammetric measurement of the dissolved metallic component (cadmium) with a disposable screen-printed electrode, which is embedded underneath the membrane on the test zone. The new device coupled with a portable electrochemical analyzer shows great promise for in-field and point-of-care quantitative testing of disease-related protein biomarkers. The parameters (e.g., voltammetric measurement of QD labels, antibody immobilization, the loading amount of QD-antibody, and the immunoreaction time) that govern the sensitivity and reproducibility of the device were optimized with IgG model analyte. The voltammetric response of the optimized device is highly linear over the range of 0.1-10 ng mL(-1) IgG, and the limit of detection is estimated to be 30 pg mL(-1) in association with a 7-min immunoreaction time. The detection limit was improved to 10 pg mL(-1) using a 20-min immunoreaction time. The device has been successfully applied for the detection of prostate-specific antigen (PSA) in human serum sample with a detection limit of 20 pg mL(-1). The results were validated by using the commercial PSA enzyme-linked immunosorbent assay kit and showed high consistency. The new disposable electrochemical diagnosis device thus provides a rapid, clinically accurate, and quantitative tool for protein biomarker detection.     

Sensitive immunoassay of a biomarker tumor necrosis factor-alpha based on poly(guanine)-functionalized silica nanoparticle label

A novel electrochemical immunosensor for the detection of tumor necrosis factor-alpha (TNF-alpha) based on poly(guanine)-functionalized silica nanoparticles (NPs) label is presented. The detection of mouse TNF-alpha via immunological reaction is based on a dual signal amplification: (1) a large amount of guanine residues introduced on the electrode surface through sandwich immunoreaction and poly(guanine)-functionalized silica NP label; (2) Ru(bpy)3(2+)-induced catalytic oxidation of guanine, which results in great enhancement of anodic current. The synthesized silica NP conjugates were characterized with atomic force microscopy, X-ray photoelectron spectroscopy, and electrochemistry. These experiments confirmed that poly(guanine) and avidin were immobilized on the surface of silica NPs. The performance of the electrochemical immunosensor was evaluated and some experiment parameters (e.g., concentration of Ru(bpy)3(2+), incubation time of TNF-alpha, etc.) were optimized. The detection limit for TNF-alpha is found to be 5.0 x 10(-11) g mL(-1) (2.0 pM), which corresponds to 60 amol of TNF-alpha in 30 microL of sample. This immunosensor based on the poly(guanine)-functionalized silica NP label offers great promise for rapid, simple, cost-effective analysis of biological samples.     

Femtomolar detection of prostate-specific antigen: an immunoassay based on surface-enhanced Raman scattering and immunogold labels

A novel reagent for low-level detection in immunoadsorbent assays is described. The reagent consists of gold nanoparticles modified to integrate bioselective species (e.g., antibodies) with molecular labels for the generation of intense, biolyte-selective surface-enhanced Raman scattering (SERS) responses in immunoassays and other bioanalytical applications. The reagent is constructed by coating gold nanoparticles (30 nm) with a monolayer of an intrinsically strong Raman scatterer. These monolayer-level labels are bifunctional by design and contain disulfides for chemisorption to the nanoparticle surface and succinimides for coupling to the bioselective species. There are two important elements in this label design; it both minimizes the separation between label and particle surface and maximizes the number of labels on each particle. This approach to labeling also exploits several other advantages of SERS-based labels: narrow spectral bandwidth, resistance to photobleaching and quenching, and long-wavelength excitation of multiple labels with a single excitation source. The strengths of this strategy are demonstrated in the detection of free prostate-specific antigen (PSA) using a sandwich assay format based on monoclonal antibodies. Detection limits of approximately 1 pg/mL in human serum and approximately 4 pg/mL in bovine serum albumin have been achieved with a spectrometer readout time of 60 s. The extension of the method to multianalyte assays (e.g., the simultaneous determination of the many complexed forms of PSA) is discussed.     

Highly sensitive photoelectrochemical immunoassay with enhanced amplification using horseradish peroxidase induced biocatalytic precipitation on a CdS quantum dots multilayer electrode

Herein we demonstrate the protocol of a biocatalytic precipitation (BCP)-based sandwich photoelectrochemical (PEC) horseradish peroxidase (HRP)-linked immunoassay on the basis of their synergy effect for the ultrasensitive detection of mouse IgG (antigen, Ag) as a model protein. The hybrid film consisting of oppositely charged polyelectrolytes and CdS quantum dots (QDs) is developed by the classic layer by layer (LbL) method and then employed as the photoactive antibody (Ab) immobilization matrix for the subsequent sandwich-type Ab-Ag affinity interactions. Improved sensitivity is achieved through using the bioconjugates of HRP-secondary antibodies (Ab(2)). In addition to the much enhanced steric hindrance compared with the original one, the presence of HRP would further stimulate the BCP onto the electrode surface for signal amplification, concomitant to a competitive nonproductive absorption that lowers the photocurrent intensity. As a result of the multisignal amplification in this HRP catalyzed BCP-based PEC immunoassay, it possesses excellent analytical performance. The antigen could be detected from 0.5 pg/mL to 5.0 ng/mL with a detection limit of 0.5 pg/mL.     

Simultaneous detection of single molecules and singulated ensembles of molecules enables immunoassays with broad dynamic range

We report a method for combining the detection of single molecules (digital) and an ensemble of molecules (analog) that is capable of detecting enzyme label from 10(-19) M to 10(-13) M, for use in high sensitivity enzyme-linked immunosorbent assays (ELISA). The approach works by capturing proteins on microscopic beads, labeling the proteins with enzymes using a conventional multistep immunosandwich approach, isolating the beads in an array of 50-femtoliter wells (Single Molecule Array, SiMoA), and detecting bead-associated enzymatic activity using fluorescence imaging. At low concentrations of proteins, when the ratio of enzyme labels to beads is less than ～1.2, beads carry either zero or low numbers of enzymes, and protein concentration is quantified by counting the presence of "on" or "off" beads (digital regime). (1) At higher protein concentrations, each bead typically carries multiple enzyme labels, and the average number of enzyme labels present on each bead is quantified from a measure of the average fluorescence intensity (analog regime). Both the digital and analog concentration ranges are quantified by a common unit, namely, average number of enzyme labels per bead (AEB). By combining digital and analog detection of singulated beads, a linear dynamic range of over 6 orders of magnitude to enzyme label was achieved. Using this approach, an immunoassay for prostate specific antigen (PSA) was developed. The combined digital and analog PSA assay provided linear response over approximately four logs of concentration ([PSA] from 8 fg/mL to 100 pg/mL or 250 aM to 3.3 pM). This approach extends the dynamic range of ELISA from picomolar levels down to subfemtomolar levels in a single measurement.     

Double-codified gold nanolabels for enhanced immunoanalysis

A novel double-codified nanolabel (DC-AuNP) based on gold nanoparticle (AuNP) modified with anti-human IgG peroxidase (HRP)-conjugated antibody is reported. It represents a simple assay that allows enhanced spectrophotometric and electrochemical detection of antigen human IgG as a model protein. The method takes advantage of two properties of the DC-AuNP label: first, the HRP label activity toward the OPD chromogen that can be related to the analyte concentration and measured spectrophotometrically; second, the intrinsic electrochemical properties of the gold nanoparticle labels that being proportional to the protein concentration can be directly quantified by stripping voltammetry. Beside these two main direct determinations of human IgG, a secondary indirect detection was also applicable to this system, exploiting the high molar absorptivity of gold colloids, by which, the color intensity of their solution was proportional to the concentration of the antigen used in the assay. Paramagnetic beads were used as supporting material to immobilize the sandwich-type immunocomplexes resulting in incubation and washing times shorter than those typically needed in classical ELISA tests by means of a rapid magnetic separation of the unbound components. A built-in magnet graphite-epoxy-composite electrode allowed a sensibly enhanced adsorption and electrochemical quantification of the specifically captured AuNPs. The used DC-AuNP label showed an excellent specificity/selectivity, as a matter of fact using a different antigen (goat IgG) a minimal nonspecific electrochemical or spectrophotometric signal was measured. The detection limits for this novel double-codified nanoparticle-based assay were 52 and 260 pg of human IgG/mL for the spectrophotometric (HRP-based) and electrochemical (AuNP-based) detections, respectively, much lower than those typically achieved by ELISA tests. The developed label and method is versatile, offers enhanced performances, and can be easily extended to other protein detection schemes as well as in DNA analysis.     

Synthesis and characterization of europium(III) nanoparticles for time-resolved fluoroimmunoassay of prostate-specific antigen

Recent advances in the fabrication and bioconjugation of nanometre-sized lanthanide(III) chelate particles have led to robust high specific activity labels. This paper describes the synthesis and characterization of lanthanide(III) nanoparticle labels and the use of a nanoparticle in a bioaffinity assay system. Two europium(III) nanoparticles were prepared using an extremely simple, inexpensive and fast agglomeration strategy. A?silica-stabilized nanoparticle was synthesized from hydrophobic tris(dibenzoylmethane)-mono(phenanthroline) and tris(dibenzoylmethane)-mono(5-aminophenanthroline) europium(III) chelates in aqueous solution. In addition, a naphthoyl trifluoroacetone:tri-n-octylphosphineoxide:sodium dodecyl sulfate europium(III) complex was agglomerated in water. The particle sizes ranged from 62 to 140?nm in diameter. The silica-stabilized particle was further coated with a monoclonal antibody. The analytical performance of the bioconjugated nanoparticle label was evaluated in a model sandwich immunoassay of prostate-specific antigen. The detection limit of human prostate-specific antigen was 28?ng?l(-1), 850?fM, in a microtiter plate format using time-resolved fluorometry. The coefficient of variation ranged from 1 to 9%. The novel nanoparticle label improves the specific activity of existing lanthanide(III) nanoparticle labels and simplifies the preparation route. In addition, prepared high-density nanoparticle labels using lanthanide(III) chelates or other specific fluorochromes have potential applications in a number of other fields.     

Ultrasensitive detection of cancer biomarkers in the clinic by use of a nanostructured microfluidic array

Multiplexed biomarker protein detection holds unrealized promise for clinical cancer diagnostics due to lack of suitable measurement devices and lack of rigorously validated protein panels. Here we report an ultrasensitive electrochemical microfluidic array optimized to measure a four-protein panel of biomarker proteins, and we validate the protein panel for accurate oral cancer diagnostics. Unprecedented ultralow detection into the 5-50 fg·mL(-1) range was achieved for simultaneous measurement of proteins interleukin 6 (IL-6), IL-8, vascular endothelial growth factor (VEGF), and VEGF-C in diluted serum. The immunoarray achieves high sensitivity in 50 min assays by using off-line protein capture by magnetic beads carrying 400,000 enzyme labels and ~100,000 antibodies. After capture of the proteins and washing to inhibit nonspecific binding, the beads are magnetically separated and injected into the array for selective capture by antibodies on eight nanostructured sensors. Good correlations with enzyme-linked immunosorbent assays (ELISA) for protein determinations in conditioned cancer cell media confirmed the accuracy of this approach. Normalized means of the four protein levels in 78 oral cancer patient serum samples and 49 controls gave clinical sensitivity of 89% and specificity of 98% for oral cancer detection, demonstrating high diagnostic utility. The low-cost, easily fabricated immunoarray provides a rapid serum test for diagnosis and personalized therapy of oral cancer. The device is readily adaptable to clinical diagnostics of other cancers.     

Electrochemical immunosensor for cholera toxin using liposomes and poly(3,4-ethylenedioxythiophene)-coated carbon nanotubes

A sensitive method for the detection of cholera toxin (CT) using an electrochemical immunosensor with liposomic magnification followed by adsorptive square-wave stripping voltammetry is described. Potassium ferrocyanide-encapsulated and ganglioside (GM1)-functionalized liposomes act as highly specific recognition labels for the amplified detection of cholera toxin. The sensing interface consists of monoclonal antibody against the B subunit of CT that is linked to poly(3,4-ethylenedioxythiophene) coated on Nafion-supported multiwalled carbon nanotube caste film on a glassy carbon electrode. The CT is detected by a "sandwich-type" assay on the electronic transducers, where the toxin is first bound to the anti-CT antibody and then to the GM1-functionalized liposome. The potassium ferrocyanide molecules are released from the bounded liposomes on the electrode by lyses with methanolic solution of Triton X-100. The released electroactive marker is measured by adsorptive square-wave stripping voltammetry. The sandwich assay provides the amplification route for the detection of the CT present in ultratrace levels. The calibration curve for CT had a linear range of 10(-14)-10(-7)g mL(-1). The detection limit of this immunosensor was 10(-16) g of cholera toxin (equivalent to 100 microL of 10(-15) g mL(-1)).     

Functionalized graphene oxide as a nanocarrier in a multienzyme labeling amplification strategy for ultrasensitive electrochemical immunoassay of phosphorylated p53 (S392)

P53 phosphorylation plays an important role in many biological processes and might be used as a potential biomarker in clinical diagnoses. We report a new electrochemical immunosensor for ultrasensitive detection of phosphorylated p53 at Ser392 (phospho-p53(392)) based on graphene oxide (GO) as a nanocarrier in a multienzyme amplification strategy. Greatly enhanced sensitivity was achieved by using the bioconjugates featuring horseradish peroxidase (HRP) and p53(392) signal antibody (p53(392)Ab(2)) linked to functionalized GO (HRP-p53(392)Ab(2)-GO) at a high ratio of HRP/p53(392)Ab(2). After a sandwich immunoreaction, the HRP-p53(392)Ab(2)-GO captured onto the electrode surface produced an amplified electrocatalytic response by the reduction of enzymatically oxidized thionine in the presence of hydrogen peroxide. The increase of response current was proportional to the phospho-p53(392) concentration in the range of 0.02-2 nM with the detection limit of 0.01 nM, which was 10-fold lower than that of the traditional sandwich electrochemical measurement for p53(392). The amplified immunoassay developed in this work shows acceptable stability and reproducibility, and the assay results for phospho-p53(392) spiked in human plasma also show good recovery (92-103.8%). This simple and low-cost immunosensor shows great promise for detection of other phosphorylated proteins and clinical applications.     

Dual-mode protein detection based on Fe<Subscript>3</Subscript>O<Subscript>4</Subscript>-Au hybrid nanoparticles

A facile method of synthesizing Fe₃O₄-Au hybrid nanoparticles is reported utilizing the multifunctional nature of polyethyleneimine (PEI). An abundance of 5 nm gold nanoparticles were attached to 50 nm Fe₃O₄ nanoparticles via the covalent binding between the -NH₂ groups of the PEI and Au nanoparticles, as well as the electrostatic interaction between the negatively charged citrate-coated Au nanoparticles and the positively charged PEI-coated Fe₃O₄ nanoparticles. The as-prepared Fe₃O₄-Au hybrid nanoparticles, which combine the merits of magnetic materials and gold, were successfully employed for the first time in the dual-mode detection of carcinoembryonic antigen (CEA) via electrochemical and surface-enhanced Raman scattering (SERS) methods. Both methods make clever use of Fe₃O₄-Au nanoparticles and can accurately verify the presence of antigens. In particular, the electrochemical immunosensor detection displays a wide linear range (0.01–10 ng/mL) of response with a low detection limit (10 pg/mL), while the SERS method responds to even lower antigen concentrations with a wider detection range. The Fe₃O₄-Au hybrid nanoparticles therefore exhibit great potential for biomedical applications.      

Facile synthesis of Ag dendrites on Al foil via galvanic replacement reaction with [Ag(NH3)2]Cl for ultrasensitive SERS detecting of biomolecules

Symmetric silver dendrites have been synthesized on commercial aluminum foil via galvanic replacement reaction with [Ag(NH₃)₂]Cl. This process is facile and environmentally friendly, without the use of any templates, surfactants or oxidants, and also avoiding the introduction of fluoride anions as a strong toxicity resulting in hypocalcemia. The products were characterized with scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution TEM (HRTEM) and X-ray diffraction (XRD). SEM characterizations and electrochemical measurements including an electrochemical direct current polarization method and OCP-t technique demonstrate that chloride has proven to be the key factor to the formation of well-defined dendritic shape. The as-prepared Ag dendrites are developed as a surface-enhanced Raman scattering (SERS)-active platform for detection of folic acid, DNA and RNA with well resolved bands and high Raman intensities. The detection concentration for the three biomolecules reaches the level of 10⁻¹² M, and thus the symmetric silver dendrites can potentially be employed as effective SERS sensors for label-free and ultrasensitive biomolecule detection.