A group-IV ferromagnetic semiconductor: MnxGe1-x

Fabrication and structural and magnetic properties of spark plasma sintered group-IV diluted magnetic semiconductor Fe-doped SiGe alloys

Fe-doped SiGe bulk alloys are fabricated using non-equilibrium spark plasma sintering (SPS) and their structure and ferromagnetic and magneto-transport properties are investigated. X-ray diffraction and high-resolution transmission electron microscope measurements show that the obtained alloys are composed of SiGe polycrystals. Magnetization measurements reveal that the Fe-doped SiGe alloys exhibit ferromagnetism up to 259 K, and their Curie temperature increases with Fe doping concentration up to 8%. Moreover, transport measurements of the Fe-doped SiGe alloys show typical metal-insulator transition characteristics of doped semiconductors as well as anomalous Hall effect and intriguing positive-to-negative magnetoresistance, indicating that the obtained alloys are diluted magnetic semiconductors (DMSs). Our results provide insight into the SPS-prepared Fe-doped SiGe bulk alloys and may be useful for the design, fabrication, and application of group-IV DMSs.

Shape-induced crystallization of binary DNA-functionalized nanocubes

Leveraging the anisotropic shape of DNA-functionalized nanoparticles holds potential for shape-directed crystallization of a wide collection of superlattice structures. Using coarse-grained molecular dynamics simulations, we study the self-assembly of a binary mixture of cubic gold nanoparticles, which are functionalized by complementary DNA strands. We observe the spontaneous self-assembly of simple cubic (SC), plastic body-centered tetragonal (pBCT), and compositionally disordered plastic body-centered tetragonal (d-pBCT) phases due to hybridization of the DNA strands. We systematically investigate the effect of length, grafting density, as well as rigidity of the DNA strands on the self-assembly behavior of cubic nanoparticles. We measure the potential of mean force between DNA-functionalized nanocubes for varying rigidity of the DNA strands and DNA lengths. Using free-energy calculations, we find that longer and flexible DNA strands can lead to a phase transformation from SC to the pBCT phase due to a gain in entropy arising from the orientational degrees of freedom of the nanocubes in the pBCT phase. Our results may serve as a guide for self-assembly experiments on DNA-functionalized cubic nanoparticles.

Synthesis and high-temperature ferromagnetism of Fe-doped SiGe diluted magnetic semiconductor thin films

Fe-doped SiGe (Si_0.25Ge_0.75:Fe_x, x = 0.01, 0.025, and 0.05) thin films were prepared by radio frequency magnetron sputtering and subsequent rapid thermal annealing on a Ge (100) substrate and their structural, magnetic and magneto-transport properties were investigated. Structural characterization using AFM, SEM, XRD, and HRTEM shows that the obtained samples are polycrystalline and their lattice constants increase with the Fe concentration. Analysis of their electronic and spintronic states using XPS and XMCD reveals that Fe dopants mainly exist as substitutional Fe²⁺ ions in the SiGe lattice, providing both local magnetic moments and hole carriers. Furthermore, magnetization measurements indicate that all the samples exhibit ferromagnetism, and their Curie temperature increases with the Fe concentration up to 294 K; meanwhile, magneto-transport measurements reveal a giant magnetoresistance (GMR) effect of over 800% and an anomalous Hall effect (AHE), as well as semiconducting behaviors, in the samples. Further analysis suggests that the ferromagnetism comes from a hole-mediated process originating from substitutional Fe dopants in the SiGe matrix and this is enhanced by the tensile strain in the films. The synthesis and high-temperature ferromagnetism of Fe-doped SiGe thin films may play a key role in group IV-based spintronic applications.

Investigation of highly ferromagnetic Mn2Ge4 and Mn2Ge5 clusters via photoelectron spectroscopy and theoretical calculations

We investigate the structures and properties of Mn₂Ge₄^-/0 and Mn₂Ge₅^-/0 by anion photoelectron spectroscopy and theoretical calculations. The vertical detachment energies (VDEs) of Mn₂Ge₄^- and Mn₂Ge₅^- are measured to be 2.69 eV and 2.49 eV, respectively. It is found that neutral Mn₂Ge₄ has an approximate quadrilateral bipyramidal structure with C_2v symmetry and ¹¹B₂ electronic state. Neutral Mn₂Ge₅ has a pentagonal bipyramidal structure with C_2v symmetry and ¹¹B₂ electronic state. The 4s-based molecular orbitals of the Mn atoms participate in the chemical bonding with the Ge₄ and Ge₅ fragments in Mn₂Ge₄ and Mn₂Ge₅. In Mn₂Ge₄, the two Mn atoms interact with the Ge₄ moiety via four GeGeMn 3c-2e σ bonds. In Mn₂Ge₅, the two Mn atoms interact with the Ge₅ moiety via one MnGeMn 3c-2e σ bond and four GeMnGe 3c-2e σ bonds. The analysis of magnetic properties reveals that both Mn₂Ge₄ and Mn₂Ge₅ exhibit highly ferromagnetic characteristics with a magnetic moment of 10 μ_B which mainly originated from the Mn atoms. These double Mn atom doped germanium clusters may provide new opportunities to design novel spintronic devices featuring high magnetic moments.

Escalating Ferromagnetic Order via Se-Vacancies Near Vanadium in WSe2 Monolayers

Magnetic order has been proposed to arise from a variety of defects, including vacancies, antisites, and grain boundaries, which are relevant in numerous electronics and spintronics applications. Nevertheless, its magnetism remains controversial due to the lack of structural analysis. The escalation of ferromagnetism in vanadium-doped WSe₂ monolayer is herein demonstrated by tailoring complex configurations of Se vacancies (Se_Vac ) via post heat-treatment. Structural analysis of atomic defects is systematically performed using transmission electron microscopy (TEM), enabled by the monolayer nature. Temperature-dependent magnetoresistance hysteresis ensures enhanced magnetic order after high-temperature heat-treatment, consistent with magnetic domain analysis from magnetic force microscopy (MFM). The vanadium-Se vacancy pairing is a key to promoting ferromagnetism via spin-flip by electron transfer, predicted from density-functional-theory (DFT) calculations. The approach toward nanodefect engineering paves a way to overcome weak magnetic order in diluted magnetic semiconductors (DMSs) for renovating semiconductor spintronics.

High Curie Temperature Achieved in the Ferromagnetic MnxGe1-x/Si Quantum Dots Grown by Ion Beam Co-Sputtering

Ferromagnetic semiconductors (FMSs) exhibit great potential in spintronic applications. It is believed that a revolution of microelectronic techniques can take off, once the challenges of FMSs in both the room-temperature stability of the ferromagnetic phase and the compatibility with Si-based technology are overcome. In this article, the Mn_xGe_1−x/Si quantum dots (QDs) with the Curie temperature (T_C) higher than the room temperature were grown by ion beam co-sputtering (IBCS). With the Mn doping level increasing, the ripening growth of MnGe QDs occurs due to self-assembly via the Stranski–Krastanov (SK) growth mode. The surface-enhanced Raman scattering effect of Mn sites observed in MnGe QDs are used to reveal the distribution behavior of Mn atoms in QDs and the Si buffer layer. The Curie temperature of Mn_xGe_1−x QDs increases, then slightly decreases with increasing the Mn doping level, and reaches its maximum value of 321 K at the doping level of 0.068. After a low-temperature and short-time annealing, the T_C value of Mn_0.068Ge_0.932 QDs increases from 321 K to 383 K. The higher Ge composition and residual strain in the IBCS grown Mn_xGe_1−x QDs are proposed to be responsible for maintaining the ferromagnetic phase above room temperature.

Application strategies of peptide nucleic acids toward electrochemical nucleic acid sensors

Peptide nucleic acids (PNAs) have attracted tremendous interest in the fabrication of highly sensitive electrochemical nucleic acid biosensors due to their higher stability and increased sensitivity than common DNA probes. The neutral pseudopeptide backbone of PNAs not only makes the PNA/DNA duplexes more stable but also provides many opportunities to construct ultrasensitive nucleic acid sensors. This review presents the details of various protocols for the construction of PNA-based electrochemical nucleic acid sensors. The crucial factors, origin, and development of PNA, immobilization methods of PNA probes and signal generation mechanisms, are discussed. This review aims to provide a reference for ultrasensitive PNA electrochemical biosensor preparation.

Room-Temperature Ferromagnetism in Mg1-xMn2+xAs2 with Layered Structure

A series of Mg/Mn mixed intermetallic compounds Mg_1-xMn_2+xAs₂ (x = 0.17, 0.48, 0.69) were synthesized by using metal flux reactions. Single-crystal X-ray diffraction measurements indicated that CaAl₂Si₂-type phases with Mn and Mg atoms located on the cation sites (Wickoff site: 1a) were obtained. The special structure of these Mg_1-xMn_2+xAs₂ compounds corresponded to unique magnetic behavior, which led to increased divergence between zero-field-cooling (ZFC) and field-cooling magnetic susceptibilities with decreasing temperature. The small magnetic hysteresis loop measured at 300 K for Mg_0.31(2)Mn_2.69As₂ revealed its room-temperature ferromagnetism, and its ZFC exchange bias behavior at low temperatures indicated the existence of both ferromagnetic (FM) and antiferromagnetic (AFM) interactions. Spin-polarized density functional theory calculations were also performed to verify the magnetic ground state, and these were consistent with the experimental results.

Optimizing the dynamic and thermodynamic properties of hybridization in DNA-mediated nanoparticle self-assembly

DNA-directed nanoparticle (DNA-NP) systems provide various applications in sensing, medical diagnosis, data storage, plasmonics and photovoltaics. Bonding probability and melting properties are helpful to evaluate the selectivity, thermostability and thermosensitivity of these applications. We investigated the influence of temperature, nanoparticle size, DNA chain length and surface grafting density of DNA on one nanoparticle on the DNA dynamic hybridization percentage and melting properties of DNA-NP assembly systems by molecular dynamics simulation. The high degree of consistency of free energy estimations for DNA hybridization via our theoretical deduction and the nearest-neighbor rule generally used in experiments validates reasonably our DNA model. The melting temperature and thermosensitivity parameter are determined by the sigmoidal melting curves based on hybridization percentage versus temperature. The results indicated that the hybridization percentage presents a downward trend with increasing temperature and nanoparticle size. Applications based on DNA-NP systems with bigger nanoparticle size, such as DNA probes, have better selectivity, thermostability and thermosensitivity. There exist optimal DNA chain length and surface grafting density where the hybridization percentage reaches the maximal value. The melting temperature reaches a maximum at the point of optimal grafting density, while the thermosensitivity parameter presents an upward trend with the increase of grafting density. Several physical quantities consisting of the radial density function, root mean square end-to-end distance, contact distance parameter and effective volume fraction are used to analyse DNA chain conformations and DNA-NP packing in the assembly process. Our findings provide the theoretical basis for the improvement and optimization of applications based on DNA-NP systems.

Electrochemically switchable polymerization from surface-anchored molecular catalysts

Redox-switchable polymerizations of lactide and epoxides were extended to the solid state by anchoring an iron-based polymerization catalyst to TiO2 nanoparticles. The reactivity of the molecular complexes and their redox-switching characteristics were maintained in the solid-state. These properties resulted in surface-initiated polymerization reactions that produced polymer brushes whose chemical composition is dictated by the oxidation state of the iron-based complex. Depositing the catalyst-functionalized TiO2 nanoparticles on fluorine-doped tin oxide resulted in an electrically addressable surface that could be used to demonstrate spatial control in redox-switchable polymerization reactions. By using a substrate that contained two electrically isolated domains wherein one domain was exposed to an oxidizing potential, patterns of surface-bound polyesters and polyethers were accessible through sequential application of lactide and cyclohexene oxide. The differentially functionalized surfaces demonstrated distinct physical properties that illustrated the promise for using the method to pattern surfaces with multiple, chemically distinct polymer brushes.

Polyphenol-Containing Nanoparticles: Synthesis, Properties, and Therapeutic Delivery

Polyphenols, the phenolic hydroxyl group-containing organic molecules, are widely found in natural plants and have shown beneficial effects on human health. Recently, polyphenol-containing nanoparticles have attracted extensive research attention due to their antioxidation property, anticancer activity, and universal adherent affinity, and thus have shown great promise in the preparation, stabilization, and modification of multifunctional nanoassemblies for bioimaging, therapeutic delivery, and other biomedical applications. Additionally, the metal-polyphenol networks, formed by the coordination interactions between polyphenols and metal ions, have been used to prepare an important class of polyphenol-containing nanoparticles for surface modification, bioimaging, drug delivery, and disease treatments. By focusing on the interactions between polyphenols and different materials (e.g., metal ions, inorganic materials, polymers, proteins, and nucleic acids), a comprehensive review on the synthesis and properties of the polyphenol-containing nanoparticles is provided. Moreover, the remarkable versatility of polyphenol-containing nanoparticles in different biomedical applications, including biodetection, multimodal bioimaging, protein and gene delivery, bone repair, antibiosis, and cancer theranostics is also demonstrated. Finally, the challenges faced by future research regarding the polyphenol-containing nanoparticles are discussed.

Self-Assembly of DNA-Functionalized Nanoparticles Guided by Binding Kinetics

We use a coarse-grained model of DNA-functionalized particles (DFPs) to understand the role of DNA strand length on their self-assembly. We find that the increasing strand length for a given particle size decreases the propensity to form ordered crystalline assemblies within the simulation time. Instead, disordered structures form when the strand length exceeds a certain threshold, consistent with the previous experiments. Analysis of the simulation data based on a pair of DFPs suggests weakening interparticle interactions with increasing strand length, thereby shifting the suitable assembly conditions to lower temperatures. We find that DNA (un)hybridization kinetics at these lower temperatures becomes significantly slower, preventing systems with longer DNA strands from crystallizing successfully. We suggest that a suitable strategy to overcome this kinetic barrier is to enhance interparticle interactions for DFPs with longer DNA strands, which is achieved by increasing the DNA grafting density. We directly test this hypothesis and show successful crystallization within the simulation time for DFPs with longer strands with higher grafting densities. Our results highlight the power of computational modeling in elucidating the fundamental design principles and guiding the assembly of nanoparticles to form complex nanostructures in cases where experiments alone have not been able to do so.

Amorphous nonstoichiometric oxides with tunable room-temperature ferromagnetism and electrical transport

Material functionalities strongly depend on the stoichiometry, crystal structure, and homogeneity. Here we demonstrate an approach of amorphous nonstoichiometric inhomogeneous oxides to realize tunable ferromagnetism and electrical transport at room temperature. In order to verify the origin of the ferromagnetism, we employed a series of structural, chemical, and electronic state characterizations. Combined with electron microscopy and transport measurements, synchrotron-based grazing incident wide angle X-ray scattering, soft X-ray absorption and circular dichroism clearly reveal that the room-temperature ferromagnetism originates from the In_0.23Co_0.77O_1-v amorphous phase with a large tunable range of oxygen vacancies. The room-temperature ferromagnetism is tunable from a high saturation magnetization of 500 emu cm^-3 to below 25 emu cm^-3, with the evolving electrical resistivity from 5 × 10³ μΩ cm to above 2.5 × 10⁵ μΩ cm. Inhomogeneous nano-crystallization emerges with decreasing oxygen vacancies, driving the system towards non-ferromagnetism and insulating regime. Our work unfolds the novel functionalities of amorphous nonstoichiometric inhomogeneous oxides, which opens up new opportunities for developing spintronic materials with superior magnetic and transport properties.

Synthesis of silver nanoparticles (AgNPs) using culinary banana peel extract for the detection of melamine in milk

Melamine adulteration in milk is a serious health concern for the consumers. A reliable and sensitive technique using silver nanoparticle (AgNPs) was developed for the detection of melamine in milk sample. The AgNPs was synthesized using culinary banana peel extract (BPE) where pH, temperature, the amount of concentration of BPE and concentration of AgNO₃ was standardized. The effect of the parameters used for the synthesis of AgNPs was analyzed by observing the colour of reaction mixture and surface plasmon resonance. The AgNPs synthesized under optimum conditions were characterized by SEM-EDX, TEM and FTIR. FTIR studies reveal the effective conjugation between AgNPs and bioactive components of BPE and formation of spherical and regular shaped AgNPs were confirmed by TEM images. Presence of Ag as a dominating metal in AgNPs confirmed the formation of AgNPs. The level of melamine above 0.5 mg/L in milk could easily be detected through the interference synthesis of AgNPs.

Influence of the type of carrier on ferromagnetism in a Si semiconductor implanted with Cu ions

Silicon semiconductor samples implanted with Cu ions and samples co-implanted with Cu- and N-ions were prepared by MEVVA and the Kaufman technique. None of the samples showed evidence of secondary phases. The initially n-type Si matrix, when implanted with Cu ions, changed to a p-type semiconductor, and the Cu ions existed as local Cu2+ cations in the p-type environment. As a result, none of the Cu-implanted samples were ferromagnetic at room temperature. The co-implanted samples, on the other hand, showed room-temperature ferromagnetism because the introduction of N ions made the carrier type change from p-type to n-type which is favorable for the appearance of Cu2+. First principles calculations were applied to understand the experimental phenomena. The formation energy was reduced by implanting N ions, and was decreased effectively with the increase in ratio of N to Cu ions. The density of states and spin density of states indicated that the hybridization of s, p and d electrons induced ferromagnetism at 0 K. Particularly, we proposed possible exchange interactions between the Cu2+-N-(N4+)-Cu2+ ions to explain the ferromagnetism mechanism.

GexSi1-x virtual-layer enhanced ferromagnetism in self-assembled Mn0.06Ge0.94 quantum dots grown on Si wafers by molecular beam epitaxy

Self-assembled Mn_0.06Ge_0.94 quantum dots (QDs) on a Si substrate or Ge_xSi_1-x virtual substrate (VS) were grown by molecular beam epitaxy. The Ge_xSi_1-x VS of different thicknesses and Ge compositions x were utilized to modulate the ferromagnetic properties of the above QDs. The MnGe QDs on Ge_xSi_1-x VS show a significantly enhanced ferromagnetism with a Curie temperature above 220 K. On the basis of the microstructural and magnetization results, the ferromagnetic properties of the QDs on Ge_xSi_1-x VS are believed to come from the intrinsic MnGe ferromagnetic phase rather than any intermetallic ferromagnetic compounds of Mn and Ge. At the same time, we found that by increasing the Ge composition x of Ge_xSi_1-x VS, the ferromagnetism of QDs grown on VS will markedly increase due to the improvements of hole concentration and Ge composition inside the QDs. These results are fundamentally important in the understanding and especially in the realization of high Curie temperature MnGe diluted magnetic semiconductors.

Oriented Immobilization on Gold Nanoparticles of a Recombinant Therapeutic Zymogen

Direct immobilization of functional proteins on gold nanoparticles (AuNPs) affects their structure and function. Changes may vary widely and range from strong inhibition to the enhancement of protein function. More often though the outcome of direct protein immobilization results in protein misfolding and the loss of protein activity. Additional complications arise when the protein being immobilized is a zymogen which requires and relies on additional protein-protein interactions to exert its function. Here we describe molecular design of a glutathione-S-transferase-Staphylokinase fusion protein (GST-SAK) and its conjugation to AuNPs. The multivalent AuNP-(GST-SAK)n complexes generated show plasminogen activation activity in vitro. The methods described are transferable and could be adapted for conjugation and functional analysis of other plasminogen activators, thrombolytic preparations or other functional enzymes.

Pyroelectric synthesis of Au/Pt bimetallic nanoparticles–BaTiO3 hybrid nanomaterials

This paper introduces an approach to synthesize bimetallic nanoparticles under an alternating temperature field in aqueous solution. During the synthesis, pyro-catalytic barium titanate is used as the substrate to reduce the metallic ions dispersed in the solution due to the generated charges at the surface of pyro-materials under temperature oscillation. Chloroauric acid and potassium tetrachloroplatinate are used as precursors to produce gold/platinum bimetallic nanoparticles through a pyro-catalytic process. Transmission electron microscopy characterization, in combination with energy dispersive X-ray spectroscopy mapping, demonstrates that the bimetallic nanoparticle is composed of an Au core and Au/Pt alloy shell structure. Compared to the conventional approaches, the pyroelectric synthesis approach demonstrated in this work requires no toxic reducing agents and waste heat can be used as a thermal energy source in the synthesis. Hence, it offers a potential “green” synthetic method for bimetallic nanoparticles.

A “green” synthetic approach to Au/Pt bimetallic nanoparticles under an alternating temperature field.

Interface engineering of microelectrodes toward ultrasensitive monitoring of β-amyloid peptides in cerebrospinal fluid in Alzheimer's disease

Aβ monomers directed the assembly of Cu²⁺-PEI/AuNPs-hemin nanoprobes into network aggregates on a microelectrode interface for enhanced electrochemical analysis.

Fabrication and Study on Magnetic-Optical Properties of Ni-Doped ZnO Nanorod Arrays

Zn_1-xNi_xO nanorod arrays were prepared on Si substrates by magnetron sputtering and hydrothermal methods at 100 °C. We studied the effects of doped concentration and hydrothermal growth conditions on the crystal structure, morphology, photoluminescence, and magnetic properties of Zn_1-xNi_xO nanorod arrays. The research results show that the Zn_1-xNi_xO nanorod have the hexagonal wurtzite structure without the appearance of the second phase, and all samples have a highly preferred orientation of a (002) crystal face. The Zn_1-xNi_xO nanorod arrays exhibit obvious room temperature ferromagnetism with saturation magnetization at 4.2 × 10⁻⁴ emu/g, the residual magnetization is 1.3 × 10⁻⁴ emu/g and the coercive field is 502 Oe, and also excellent luminescent properties with seven times greater luminous intensity than that of ZnO nanorod arrays. The redshift of the ultraviolet emission peak was found by Ni²⁺ doping. We further explained the source and essence of the magnetic properties of Zn_1-xNi_xO nanorod arrays and deemed that the magnetic moment mainly comes from the hybrid electron exchange of O 2p and Ni 3d state.

Feedback control for shaping density distributions of colloidal particles in microfluidic devices

Directed self-assembly has great potential for the precise manufacture of structured materials at the micro/nano-scale. A local particle density often has to be controlled to make the assembly of complicated structures with no defects attainable. However, the control of spatial particle density distributions is challenged by the need for multiple actuators, kinetic trapping and the stochastic nature of self-assembly systems. In this paper, a novel feedback control approach for shaping spatial density distributions of colloidal particles is presented. The control objective is to maintain the ratio of the particle densities of two adjacent regions close to a desired value. A microfluidic device with a triple-parallel microelectrode is fabricated to provide multiple actuators for particle manipulation. The multiple-electrode actuators can be operated flexibly to either direct particles between two adjacent regions or to maintain particles within regions by preventing undesired particle movements. A feedback control scheme is implemented to control the density ratio over a broad range of tested set points. The method is generic and can be extended to include additional parallel electrodes for the control of density distributions at higher resolutions due to a modular design.

From DNA Nanotechnology to Material Systems Engineering

In the past 35 years, DNA nanotechnology has grown to a highly innovative and vibrant field of research at the interface of chemistry, materials science, biotechnology, and nanotechnology. Herein, a short summary of the state of research in various subdisciplines of DNA nanotechnology, ranging from pure "structural DNA nanotechnology" over protein-DNA assemblies, nanoparticle-based DNA materials, and DNA polymers to DNA surface technology is given. The survey shows that these subdisciplines are growing ever closer together and suggests that this integration is essential in order to initiate the next phase of development. With the increasing implementation of machine-based approaches in microfluidics, robotics, and data-driven science, DNA-material systems will emerge that could be suitable for applications in sensor technology, photonics, as interfaces between technical systems and living organisms, or for biomimetic fabrication processes.

Programmable Atom Equivalents: Atomic Crystallization as a Framework for Synthesizing Nanoparticle Superlattices

Decades of research efforts into atomic crystallization phenomenon have led to a comprehensive understanding of the pathways through which atoms form different crystal structures. With the onset of nanotechnology, methods that use colloidal nanoparticles (NPs) as nanoscale "artificial atoms" to generate hierarchically ordered materials are being developed as an alternative strategy for materials synthesis. However, the assembly mechanisms of NP-based crystals are not always as well-understood as their atomic counterparts. The creation of a tunable nanoscale synthon whose assembly can be explained using the context of extensively examined atomic crystallization will therefore provide significant advancement in nanomaterials synthesis. DNA-grafted NPs have emerged as a strong candidate for such a "programmable atom equivalent" (PAE), because the predictable nature of DNA base-pairing allows for complex yet easily controlled assembly. This Review highlights the characteristics of these PAEs that enable controlled assembly behaviors analogous to atomic phenomena, which allows for rational material design well beyond what can be achieved with other crystallization techniques.

Biopebble Containers: DNA-Directed Surface Assembly of Mesoporous Silica Nanoparticles for Cell Studies

The development of methods for colloidal self-assembly on solid surfaces is important for many applications in biomedical sciences. Toward this goal, described is a versatile class of mesoporous silica nanoparticles (MSN) that contain on their surface various types of DNA molecules to enable their self-assembly into micropatterned surface architectures useful for cell studies. Monodisperse dye-doped MSN are synthesized by biphase stratification and functionalized with an aptamer oligonucleotide that serves as gatekeeper for the triggered release of encapsulated molecular cargo, such as fluorescent dye rhodamine B or the anticancer drug doxorubicin. One or two additional types of oligonucleotides are installed on the MSN surface to enable DNA-directed immobilization on solid substrates bearing patterns of complementary capture oligonucleotides. It is demonstrated that this strategy can be used for efficient self-assembly of microstructured surface architectures, which not only promote the adhesion and guidance of cells but also are capable of affecting the fate of adhered cells through triggered release of their cargo. It is believed that this approach is useful for diverse applications in tissue engineering and nanobio sciences.

Ultra-fast annealing manipulated spinodal nano-decomposition in Mn-implanted Ge

In the present work, millisecond-range flash lamp annealing is used to recrystallize Mn-implanted Ge. Through systematic investigations of structural and magnetic properties, we find that the flash lamp annealing produces a phase mixture consisting of spinodally decomposed Mn-rich ferromagnetic clusters within a paramagnetic-like matrix with randomly distributed Mn atoms. Increasing the annealing energy density from 46, via 50, to 56 J cm^-2 causes the segregation of Mn atoms into clusters, as proven by transmission electron microscopy analysis and quantitatively confirmed by magnetization measurements. According to x-ray absorption spectroscopy, the dilute Mn ions within Ge are in d ⁵ electronic configuration. This Mn-doped Ge shows paramagnetism, as evidenced by the unsaturated magnetic-field-dependent x-ray magnetic circular dichroism signal. Our study reveals how spinodal decomposition occurs and influences the formation of ferromagnetic Mn-rich Ge-Mn nanoclusters.

A novel electrochemical strategy based on porous 3D graphene-starch architecture and silver deposition for ultrasensitive detection of neuron-specific enolase

This work was aimed at designing a novel and ultrasensitive electrochemical immunoassay strategy to detect neuron-specific enolase (NSE) with a triple signal amplification strategy.

On-chip plasmonic immunoassay based on targeted assembly of gold nanoplasmonic particles

An on-chip, non-enzymatic immunoassay was developed via the targeted assemblies of gold nanoparticles with target proteins in degassing-driven microfluidic devices and simply quantified at the single particle level.

Functional Imaging with Nucleic-Acid-Based Sensors: Technology, Application and Future Healthcare Prospects

Timely monitoring and assessment of human health plays a crucial role in maintaining the wellbeing of our advancing society. In addition to medical tools and devices, suitable probe agents are crucial to assist such monitoring, either in passive or active ways (i.e., sensors) through inducible signals. In this review we highlight recent developments in activatable optical sensors based on nucleic acids. Sensing mechanisms and bio-applications of these nucleic acid sensors in ex vivo assays, intracellular or in vivo settings are described. In addition, we discuss the limitations of these sensors and how nanotechnology can complement/enhance sensor properties to promote translation into clinical applications.

In situ, amplification-free double-stranded mutation detection at 60 copies/ml with thousand-fold wild type in urine

We have investigated amplification-free in situ double-stranded mutation detection in urine in the concentration range 10^-19 M - 10^-16 M using piezoelectric plate sensors (PEPs). The detection was carried out in a close-loop flow with two temperature zones. The 95 °C high-temperature zone served as the reservoir where the sample was loaded and DNA de-hybridized. The heated urine was cooled flowing through a 1 m long tubing immersed in room-temperature water bath at a flow rate of 4 ml/min to reach the detection cell at the desired temperature for the detection to take place. With hepatitis B virus double mutation (HBVDM) and KRAS G12V point mutation as model double mutations, it is shown that PEPS was able to detect double-stranded HBVDM and KRAS with 70% detection efficiency or better at concentration as low as 10^-19 M against single-stranded mutation detection at the same concentrations, which was validated by the following in situ fluorescent reporter microspheres (FRMs) detection as well as microscopic visualization of the FRMs bound to the captured mutant on the PEPS surface. Furthermore, the same double-stranded mutation detection efficacy was demonstrated at 10^-19 M - 10^-16 M in a background of 250-fold wildtype for HBVDM and 1000-fold wildtype for KRAS. Also demonstrated was detection of KRAS mutation at 10^-19 M - 10^-16 M of SW480 DNA fragments in urine.

Feedback control for defect-free alignment of colloidal particles

Precise alignment of small-scale building blocks into specific structural features is important for the manufacture of novel materials. Directed self-assembly is a promising route to align such small-scale building blocks with single-particle resolution. However, reliable alignment via directed self-assembly is challenging due to design uncertainty, randomness and potential disturbances acting on the system. This paper presents an integrated feedback control strategy to align colloidal particles reliably using directed self-assembly with electric field properties as manipulated variables in a microfluidic device. First, the particle density is controlled to make assembly of a defect-free structure attainable. Subsequently, a novel control method for particle alignment is implemented to self-assemble lines with single-particle resolution. The system's ergodicity is restricted systematically to assure that the density-control step at the higher hierarchy restricts the alignment-control step at the lower hierarchy. The method exploits several electrokinetic phenomena and all steps are fully automated. The approach is generic and can in principle be extended to include more density control steps to self-assemble more complicated structures.

3D Hybrid Small Scale Devices

Interfacing nano/microscale elements with biological components in 3D contexts opens new possibilities for mimicry, bionics, and augmentation of organismically and anatomically inspired materials. Abiotic nanoscale elements such as plasmonic nanostructures, piezoelectric ribbons, and thin film semiconductor devices interact with electromagnetic fields to facilitate advanced capabilities such as communication at a distance, digital feedback loops, logic, and memory. Biological components such as proteins, polynucleotides, cells, and organs feature complex chemical synthetic networks that can regulate growth, change shape, adapt, and regenerate. Abiotic and biotic components can be integrated in all three dimensions in a well-ordered and programmed manner with high tunability, versatility, and resolution to produce radically new materials and hybrid devices such as sensor fabrics, anatomically mimetic microfluidic modules, artificial tissues, smart prostheses, and bionic devices. In this critical Review, applications of small scale devices in 3D hybrid integration, biomicrofluidics, advanced prostheses, and bionic organs are discussed.

Room-temperature ferromagnetic Cr-doped Ge/GeOx core-shell nanowires

The Cr-doped tunable thickness core-shell Ge/GeO_x nanowires (NWs) were synthesized and characterized using x-ray diffraction, field-emission scanning electron microscopy, transmission electron microscopy, energy-dispersive x-ray spectroscopy, x-ray photoelectron spectroscopy and magnetization studies. The shell thickness increases with the increase in synthesis temperature. The presence of metallic Cr and Cr³⁺ in core-shell structure was confirmed from XPS study. The magnetic property is highly sensitive to the core-shell thickness and intriguing room temperature ferromagnetism is realized only in core-shell NWs. The magnetization decreases with an increase in shell thickness and practically ceases to exist when there is no core. These NWs show remarkably high Curie temperature (T_C > 300 K) with the dominating values of its magnetic remanence (M_R) and coercivity (H_C) compared to germanium dilute magnetic semiconductor nanomaterials. We believe that our finding on these Cr-doped Ge/GeO_X core-shell NWs has the potential to be used as a hard magnet for future spintronic devices, owing to their higher characteristic values of ferromagnetic ordering.

Structure and Magnetism of Mn₅Ge₃ Nanoparticles

In this work, we investigated the magnetic and structural properties of isolated Mn₅Ge₃ nanoparticles prepared by the cluster-beam deposition technique. Particles with sizes between 7.2 and 12.6 nm were produced by varying the argon pressure and power in the cluster gun. X-ray diffraction (XRD)and selected area diffraction (SAD) measurements show that the nanoparticles crystallize in the hexagonal Mn₅Si₃-type crystal structure, which is also the structure of bulk Mn₅Ge₃. The temperature dependence of the magnetization shows that the as-made particles are ferromagnetic at room temperature and have slightly different Curie temperatures. Hysteresis-loop measurements show that the saturation magnetization of the nanoparticles increases significantly with particle size, varying from 31 kA/m to 172 kA/m when the particle size increases from 7.2 to 12.6 nm. The magnetocrystalline anisotropy constant K at 50 K, determined by fitting the high-field magnetization data to the law of approach to saturation, also increases with particle size, from 0.4 × 10⁵ J/m³ to 2.9 × 10⁵ J/m³ for the respective sizes. This trend is mirrored by the coercivity at 50 K, which increases from 0.04 T to 0.13 T. A possible explanation for the magnetization trend is a radial Ge concentration gradient.

Innovations in biomedical nanoengineering: nanowell array biosensor

Nanostructured biosensors have pioneered biomedical engineering by providing highly sensitive analyses of biomolecules. The nanowell array (NWA)-based biosensing platform is particularly innovative, where the small size of NWs within the array permits extremely profound sensing of a small quantity of biomolecules. Undoubtedly, the NWA geometry of a gently-sloped vertical wall is critical for selective docking of specific proteins without capillary resistances, and nanoprocessing has contributed to the fabrication of NWA electrodes on gold substrate such as molding process, e-beam lithography, and krypton-fluoride (KrF) stepper semiconductor method. The Lee group at the Mara Nanotech has established this NW-based biosensing technology during the past two decades by engineering highly sensitive electrochemical sensors and providing a broad range of detection methods from large molecules (e.g., cells or proteins) to small molecules (e.g., DNA and RNA). Nanosized gold dots in the NWA enhance the detection of electrochemical biosensing to the range of zeptomoles in precision against the complementary target DNA molecules. In this review, we discuss recent innovations in biomedical nanoengineering with a specific focus on novel NWA-based biosensors. We also describe our continuous efforts in achieving a label-free detection without non-specific binding while maintaining the activity and stability of immobilized biomolecules. This research can lay the foundation of a new platform for biomedical nanoengineering systems.

Significance of DNA bond strength in programmable nanoparticle thermodynamics and dynamics

Assembly of nanoparticles (NPs) coated with complementary DNA strands leads to novel crystals with nanosized basic units rather than classic atoms, ions or molecules. The assembly process is mediated by hybridization of DNA via specific base pairing interaction, and is kinetically linked to the disassociation of DNA duplexes. DNA-level physiochemical quantities, both thermodynamic and kinetic, are key to understanding this process and essential for the design of DNA-NP crystals. The melting transition properties are helpful to judge the thermostability and sensitivity of relative DNA probes or other applications. Three different cases are investigated by changing the linker length and the spacer length on which the melting properties depend using the molecular dynamics method. Melting temperature is determined by sigmoidal melting curves based on hybridization percentage versus temperature and the Lindemann melting rule simultaneously. We provide a computational strategy based on a coarse-grained model to estimate the hybridization enthalpy, entropy and free energy from percentages of hybridizations which are readily accessible in experiments. Importantly, the lifetime of DNA bond dehybridization based on temperature and the activation energy depending on DNA bond strength are also calculated. The simulation results are in good agreement with the theoretical analysis and the present experimental data. Our study provides a good strategy to predict the melting temperature which is important for the DNA-directed nanoparticle system, and bridges the dynamics and thermodynamics of DNA-directed nanoparticle systems by estimating the equilibrium constant from the hybridization of DNA bonds quantitatively.

Magnetic moment and magnetic anisotropy of Ge3Mn5 thinfilms on Ge(111) substrate: A density functional study

Magnetism and magnetic anisotropy energy (MAE) of the Ge₃Mn₅ bulk, free-standing surface, and Ge₃Mn₅(001)|Ge(111) thinfilms and superlattice have been systemically investigated by using the relativistic first-principles electronic structure calculations. For Ge₃Mn₅ adlayers on Ge(111) substrates within Mn1 termination, the direction of magnetization undergoes a transition from in-plane at 1 monolayer (ML) thickness (MAE = -0.50 meV/ML) to out-of-plane beginning at 3 ML thickness (nearly invariant MAE = 0.16 meV/ML). The surficial/interfacial MAE is extracted to be 1.23/-0.54 meV for Mn1-termination; the corresponding value is 0.19/1.03 meV for Mn2/Ge-termination; the interior MAE is averaged to be 0.09 meV per ML. For various Ge₃Mn₅ systems, the in-plane lattice expansion and/or interlayer distance contraction would enhance the out-of-plane MAE. Our theoretical magnetic moments and MAEs fit well with the experimental measurements. Finally, the origination of MAE is elucidated under the framework of second-order perturbation with the electronic structure analyses.

Identifying DNA mismatches at single-nucleotide resolution by probing individual surface potentials of DNA-capped nanoparticles

Here, we demonstrate a powerful method to discriminate DNA mismatches at single-nucleotide resolution from 0 to 5 mismatches (χ₀ to χ₅) using Kelvin probe force microscopy (KPFM). Using our previously developed method, we quantified the surface potentials (SPs) of individual DNA-capped nanoparticles (DCNPs, ∼100 nm). On each DCNP, DNA hybridization occurs between ∼2200 immobilized probe DNA (pDNA) and target DNA with mismatches (tDNA, ∼80 nM). Thus, each DCNP used in the bioassay (each pDNA-tDNA interaction) corresponds to a single ensemble in which a large number of pDNA-tDNA interactions take place. Moreover, one KPFM image can scan at least dozens of ensembles, which allows statistical analysis (i.e., an ensemble average) of many bioassay cases (ensembles) under the same conditions. We found that as the χ_n increased from χ₀ to χ₅ in the tDNA, the average SP of dozens of ensembles (DCNPs) was attenuated owing to fewer hybridization events between the pDNA and the tDNA. Remarkably, the SP attenuation vs. the χ_n showed an inverse-linear correlation, albeit the equilibrium constant for DNA hybridization exponentially decreased asymptotically as the χ_n increased. In addition, we observed a cascade reaction at a 100-fold lower concentration of tDNA (∼0.8 nM); the average SP of DCNPs exhibited no significant decrease but rather split into two separate states (no-hybridization vs. full-hybridization). Compared to complementary tDNA (i.e., χ₀), the ratio of no-hybridization/full-hybridization within a given set of DCNPs became ∼1.6 times higher in the presence of tDNA with single mismatches (i.e., χ₁). The results imply that our method opens new avenues not only in the research on the DNA hybridization mechanism in the presence of DNA mismatches but also in the development of a robust technology for DNA mismatch detection.

DNA metallization: principles, methods, structures, and applications

This review summarizes the research activities on DNA metallization since the concept was first proposed in 1998, covering the principles, methods, structures, and applications.

Development of quantum dot-based biosensors: principles and applications

We review the recent advances in quantum dot-based biosensors and focus on quantum dot-based fluorescent, bioluminescent, chemiluminescent, and photoelectrochemical biosensors.

Nanomaterial-based devices for point-of-care diagnostic applications

In this review, we have discussed the capabilities of nanomaterials for point-of-care (PoC) diagnostics and explained how these materials can help to strengthen, miniaturize and improve the quality of diagnostic devices.

Thermally reversible nanoparticle gels with tuneable porosity showing structural colour

We present colloidal gels formed from dispersions of PEG- and PEG+DNA-coated silica nanoparticles showing structural colour. The PEG- and PEG+DNA-coated silica colloids are functionalized using exclusively covalent bonds in aqueous conditions. Both sets of colloids self-assemble into thermally-reversible colloidal gels with porosity that can be tuned by changing the colloid volume fraction, although the interaction potentials of the colloids in the two systems are different. Confocal microscopy and image analysis tools are used to characteraize the gels' microstructures. Optical reflection spectroscopy is employed to study the underlying gel nanostructure and to characterize the optical response of the gels. X-ray nanotomography is used to visualize the nanoscale phase separation between the colloid-rich gel branches and the colloid-free gel pores. These nanoparticle gels open new routes for creating structural colour where the gel structure is decoupled from the form factor of the individual colloids. This approach can be extended to create unexplored three dimensional macroscale materials with length scales spanning hundreds of nanometers, which has been difficult to achieve using other methods.

Glassy magnetic ground state and Kondo-like behaviour in Mn10FeGe8 alloy

We report a detailed investigation of the ground-state magnetic properties of newly synthesized [Formula: see text] [Formula: see text] alloy. The sample can be thought of being derived by substituting one Mn atom by Fe of the parent compound [Formula: see text] [Formula: see text]. Fe-substitution leads to a drastic change in the magnetic ground state as well as to the magneto-transport properties of the parent alloy. On cooling below 250 K, [Formula: see text] [Formula: see text] undergoes a transition from paramagnetic phase to a state having significant ferromagnetic correlations. The ground state is found to be canonical spin glass (CSG) type in nature as evident from the dc magnetization and ac susceptibility measurements. Interestingly, the resistivity data shows an upturn at low temperature below about 30 K, mimicking Kondo-like behaviour. [Formula: see text] [Formula: see text] turns out to be a rare example among 3d transition metal alloys, where a Kondo-like state coexists within a CSG phase.

Highly sensitive MicroRNA 146a detection using a gold nanoparticle-based CTG repeat probing system and isothermal amplification

We have developed a gold nanoparticle (AuNP)-based CTG repeat probing system displaying high quenching capability and combined it with isothermal amplification for the detection of miRNA 146a. This method of using a AuNP-based CTG repeat probing system with isothermal amplification allowed the highly sensitive (14 aM) and selective detection of miRNA 146a. A AuNP-based CTG repeat probing system having a hairpin structure and a dT^F fluorophore exhibited highly efficient quenching because the CTG repeat-based stable hairpin structure imposed a close distance between the AuNP and the dT^F residue. A small amount of miRNA 146a induced multiple copies of the CAG repeat sequence during rolling circle amplification; the AuNP-based CTG repeat probing system then bound to the complementary multiple-copy CAG repeat sequence, thereby inducing a structural change from a hairpin to a linear structure with amplified fluorescence. This AuNP-based CTG probing system combined with isothermal amplification could also discriminate target miRNA 146a from one- and two-base-mismatched miRNAs (ORN 1 and ORN 2, respectively). This simple AuNP-based CTG probing system, combined with isothermal amplification to induce a highly sensitive change in fluorescence, allows the detection of miRNA 146a with high sensitivity (14 aM) and selectivity.