pH-responsive deoxyribonucleic acid capture/release by polydopamine functionalized magnetic nanoparticles

Preloading Long-Chain Quaternary Ammonium Groups to Synthesize a High-Efficient Anion-Exchange Resin for Eliminating Bacterial Contaminants in Drinking Water

Bacterial contamination in drinking water is a global health concern, necessitating the development of highly efficient treatment techniques. Anion-exchange resins (AERs) have long been employed for removing anionic contaminants from drinking water, but their performance for bacterial contamination is poor. Here, we develop a novel AER (AER6-1) with exceptional bactericidal effects and ultrafast adsorption rates of extracellular DNA (eDNA) (2.2- and 11.5-fold compared to other AERs) achieved through preloading quaternary ammonium groups (QAGs) with hexyl chain (-C6-N+-) on the resin exterior and successively grafting QAGs with a methyl chain (-C1-N+-) inside a resin pore. The AER6-1 outperforms other commercial AERs and ultraviolet disinfection, exhibiting superior elimination of total bacteria, potential pathogens (Escherichia coli and Pseudomonas aeruginosa), eDNA, and antibiotic resistance genes (mexF, mexB, and bacA) in actual drinking water, while maintaining a comparable anion exchange capacity with other commercial AERs. Theoretical calculations of density functional theory and xDLVO combined with XPS elucidate the crucial roles of hydrogen bonding and hydrophobic force provided by the resin skeleton and -C6-N+- in cleaving the bacterial cell membrane and increasing the adsorption kinetics on eDNA. This study broadens the scope of AERs and highlights an effective way of simultaneously removing bacterial and anionic contaminants from drinking water.

Poly(3,4-dihydroxyphenylalanine)-modified cellulose paper for the extraction of deoxyribonucleic acid by a laboratory-built automated extraction device

The success of polymerase chain reaction (PCR) depends on the quality of deoxyribonucleic acid (DNA) templates. This study developed a cost-effective and eco-friendly DNA extraction system utilizing poly(3,4-dihydroxyphenylalanine)-modified cellulose paper (polyDOPA@paper). PolyDOPA@paper was prepared by oxidatively self-polymerizing DOPA under weak alkaline conditions and utilizing the adhesive property of polyDOPA on different materials. Compared to the uncoated cellulose paper, polyDOPA coating significantly enhances DNA adsorption owing to its abundant amino, carboxyl, and hydroxyl moieties. The DNA extraction mechanism using polyDOPA@paper was discussed. The maximum adsorption capacity of polyDOPA@paper for DNA was 20.7 μg cm-2. Moreover, an automated extraction system was designed and fabricated using 3D printing technology. The device simplifies the operation and ensures the reproducibility and consistency of the results. More importantly, it eliminates the need for specialized training of operators. The feasibility of the polyDOPA@paper-based automated extraction system was evaluated by quantitatively detecting Escherichia coli in spiked milk samples via a real-time PCR. The detection limit was 102 cfu mL-1. The results suggest that the system would have significant potential in detecting pathogens.

Magnetic-gold nanoparticle-mediated paper-based biosensor for highly sensitive colorimetric detection of food adulteration

Food adulteration presents a significant challenge due to the evasion of legal oversight and the difficulty of identification. Addressing this issue, there is an urgent need for on-site, rapid, visually based small-scale equipment, along with large-scale screening technology, to enable prompt results without providing opportunities for dishonest traders to react. Colorimetric reactions offer advantages in terms of speed, visualization, and miniaturization. However, there is a scarcity of suitable colorimetric reactions for food adulteration detection, and interference from colored food impurities and easily comparable color results affects accuracy. To overcome limitations, this study introduces a novel approach utilizing polydopamine magnetic nanoparticles to enrich DNA in food samples, effectively eliminating interfering components. By employing gold nanoparticles to generate magnetic-gold nanoparticles, a single magnetic bead achieves simultaneous enrichment, impurity removal, and detection. The use of paper-based biosensors and visualization equipment allows for the visualization and digital analysis of results, achieving a low detection limit of 4.59 nmol mL-1. The method also exhibits high accuracy and repeatability, with a RSD ranging from 1.6 % to 4.0 %. This innovative colorimetric method addresses the need for rapid, miniaturized, and large-scale detection, thus providing a solution for food adulteration challenges.

Isopropanol-promoted DNA extraction by polydopamine functionalized magnetic particles based on metal coordination

High-quality DNA is an important guarantee to start downstream experiments in many biological and medical research areas. Magnetic particle-based DNA extraction methods from blood mainly depend on electrostatic adsorption in a low-pH environment. However, the strong acidic environment can influence the DNA stability. Herein, a polydopamine-functionalized magnetic particle (PDA@Fe3O4)-based protocol was developed for DNA extraction from whole blood samples. In the protocol, Mg2+ and Ca2+ were utilized to bridge the adsorption of DNA by PDA@Fe3O4 via the metal-mediated coordination. Isopropanol was found to efficiently promote DNA adsorption by triggering the change of the conformation of DNA from B-form to more compact A-form. In 50 % isopropanol solution, the DNA adsorption efficiency was nearly 100 % in the presence of 0.5 mM Ca2+ or 1.5 mM Mg2+. The role of metal ions and isopropanol in DNA adsorption was explored. The protocol averts the strong acidic environment and PCR inhibitors, such as high concentrations of salt or polyethylene glycol. It demonstrates superiority in DNA yield (59.13 ± 3.63 ng μL-1) over the commercial kit (27.33 ± 4.98 ng μL-1) and phenol-chloroform methods (37.90 ± 0.47 ng μL-1). In addition, to simplify the operastion, an automated nucleic acid extraction device was designed and fabricated to extract whole genomic DNA from blood. The feasibility of the device was verified by extracting DNA from cattle and pig blood samples. The extracted DNA was successfully applied to discriminate the beef authenticity by a duplex PCR system. The results demonstrate that the DNA extraction protocol and the automated device have great potential in blood samples.

Recent Development and Applications of Polydopamine in Tissue Repair and Regeneration Biomaterials

The various tissue damages are a severe problem to human health. The limited human tissue regenerate ability requires suitable biomaterials to help damage tissue repair and regeneration. Therefore, many researchers devoted themselves to exploring biomaterials suitable for tissue repair and regeneration. Polydopamine (PDA) as a natural and multifunctional material which is inspired by mussel has been widely applied in different biomaterials. The excellent properties of PDA, such as strong adhesion, photothermal and high drug-loaded capacity, seem to be born for tissue repair and regeneration. Furthermore, PDA combined with different materials can exert unexpected effects. Thus, to inspire researchers, this review summarizes the recent and representative development of PDA biomaterials in tissue repair and regeneration. This article focuses on why apply PDA in these biomaterials and what PDA can do in different tissue injuries.

Integrated Three-Dimensional Microdevice with a Modified Surface for Enhanced DNA Separation from Biological Samples

Functional interfaces and devices for rapid adsorption and immobilization of nucleic acids (NAs) are significant for relevant bioengineering applications. Herein, a microdevice with poly(acrylic acid) (PAA) photosensitive resin was integrated by three-dimensional (3D) printing, named DPAA for short. Precise microscale structures and abundant surface carboxyl functional groups were fabricated for fast and high-throughput deoxyribonucleic acid (DNA) separation. Surface modification was then done using polydopamine (PDA) and poly(ethylene glycol) (PEG) to obtain modified poly(acrylic acid) (PAA)-based devices DPDA-PAA and DPEG-PAA rich in amino and hydroxyl groups, respectively. The fabricated device DPAA possessed superior printing accuracy (40-50 μm). Functionalization of amino and hydroxyl was successful, and the modified devices DPDA-PAA and DPEG-PAA maintained a high thermal stability like DPAA. Surface potential analysis and molecular dynamics simulation indicated that the affinity for DNA was in the order of DPDA-PAA > DPEG-PAA > DPAA. Further DNA separation experiments confirmed the high throughput and high selectivity of DNA separation performance, consistent with the predicted affinity results. DPDA-PAA showed relatively the highest DNA extraction yield, while DPEG-PAA was the worst. An acidic binding system is more favorable for DNA separation and recovery. DPDA-PAA showed significantly better DNA extraction performance than DPAA in a weakly acidic environment (pH 5.0-7.0), and the average DNA yield of the first elution was 2.16 times that of DPAA. This work validates the possibility of modification on integrated 3D microdevices to improve their DNA separation efficiency effectively. It also provides a new direction for the rational design and functionalization of bioengineering separators based on nonmagnetic methods. It may pave a new path for the highly efficient polymerase chain reaction diagnosis.

Simplified Synthesis of the Amine-Functionalized Magnesium Ferrite Magnetic Nanoparticles and Their Application in DNA Purification Method

For pathogens identification, the PCR test is a widely used method, which requires the isolation of nucleic acids from different samples. This extraction can be based on the principle of magnetic separation. In our work, amine-functionalized magnesium ferrite nanoparticles were synthesized for this application by the coprecipitation of ethanolamine in ethylene glycol from Mg(II) and Fe(II) precursors. The conventional synthesis method involves a reaction time of 12 h (MgFe2O4-H&R MNP); however, in our modified method, the reaction time could be significantly reduced to only 4 min by microwave-assisted synthesis (MgFe2O4-MW MNP). A comparison was made between the amine-functionalized MgFe2O4 samples prepared by two methods in terms of the DNA-binding capacity. The experimental results showed that the two types of amine-functionalized magnesium ferrite magnetic nanoparticles (MNPs) were equally effective in terms of their DNA extraction yield. Moreover, by using a few minutes-long microwave synthesis, we obtained the same quality magnesium ferrite particles as those made through the long and energy-intensive 12-h production method. This advancement has the potential to improve and expedite pathogen identification processes, helping to better prevent the spread of epidemics.

Immobilization of Multivalent Titanium Cations on Magnetic Composite Microspheres for Highly Efficient DNA Extraction and Amplification

Magnetic-assisted DNA testing technology has attracted much attention in genetics, clinical diagnostics, environmental microbiology, and molecular biology. However, achieving satisfying DNA adsorption and desorption efficiency in real samples is still a big challenge. In this paper, a new kind of high-quality magnetic composite microsphere of MM@PGMA-PA-Ti4+ was designed and prepared for DNA extraction and detection based on the strong interaction of Ti4+ and phosphate groups. By taking the advantages of high magnetic susceptibility and high Ti4+ content, the MM@PGMA-PA-Ti4+ microspheres possessed remarkable extraction capacity for mimic biological samples (salmon sperm specimens) with saturated loadings up to 533.0 mg/g. When the DNA feeding amount was 100 μg and the MM@PGMA-PA-Ti4+ dosage was 1 mg, the adsorption and desorption efficiencies were 80 and 90%, respectively. The kinetic and equilibrium extraction data were found to fit well with the pseudo-second-order model and Freundlich isotherm model. Furthermore, the MM@PGMA-PA-Ti4+ microspheres were successfully employed for DNA extraction from mouse epithelial-like fibroblasts. The extraction ability (84 ± 4 μg/mg) and DNA purity were superior to the comparative commercial spin kits, as evaluated by electrophoresis assays and qPCR analysis. The experimental results suggest that the MM@PGMA-PA-Ti4+ microspheres possess great potential as an adsorbent for DNA purification from complex biological samples.

Solvent-Responsive Magnetic Beads for Accurate Detection of SARS-CoV-2

Although numerous approaches were proposed for the nucleic acid (NA)-based SARS-CoV-2 detection, the nonideal NA desorption efficiency of conventional magnetic beads (MBs) limits their widespread application. In this study, we developed solvent-responsive MBs (called responsive MBs), which, in the presence of buffers, modulated the absorption and desorption capacities of NA by flipping the surface -COO-. Relative to other commercial MBs, responsive MBs exhibited similar absorption profiles and markedly enhanced desorption profiles. When applied for NA detection of complex samples, responsive MBs exhibited better performance of RNA detection than DNA, with obvious advantages in sensitivity. Specifically, the RNA and DNA desorption rates of commercial MBs were ∼85 and 82.5%, while those of responsive MBs were nearly 94 and 93.5%, respectively. Furthermore, responsive MBs exhibited remarkable extraction ability in a wide range of tissues and better performance of RNA extraction than DNA. When applied for SARS-CoV-2 detection, the responsive MBs along with the simulated digital RT-LAMP (a previously established apparatus) further improved detection efficiency, yielding a precise quantitative detection as low as 25 copies and an ultimate sensibility detection of 5 copies/mL. It was also successfully employed in numerous NA-based technologies such as polymerase chain reaction (PCR), sequencing, and so on.

Dimethylamino group modified polydopamine nanoparticles with positive charges to scavenge cell-free DNA for rheumatoid arthritis therapy

Excessive cell-free DNA (cfDNA) released by damaged or apoptotic cells can cause inflammation, impacting the progression of rheumatoid arthritis (RA). cfDNA scavengers, such as cationic nanoparticles (NPs), have been demonstrated as an efficient strategy for treating RA. However, most scavengers are limited by unfavorable biocompatibility and poor scavenging efficacy. Herein, by exploiting the favorable biocompatibility, biodegradability and bioadhesion of polydopamine (P), we modified P with dimethylamino groups to form altered charged DPs to bind negatively charged cfDNA for RA therapy. Results showed that DPs endowed with superior binding affinity of cfDNA and little cytotoxicity, which effectively inhibited lipopolysaccharide (LPS) stimulated inflammation in vitro, resulting in the relief of joint swelling, synovial hyperplasia and cartilage destruction in RA rats. Significantly, DPs with higher DS of bis dimethylamino group exhibited higher positive charge density and stronger cfDNA binding affinity, leading to excellent RA therapeutic effect among all of the treated groups, which was even close to normal rats. These finding provides a novel strategy for the treatment of cfDNA-associated diseases.

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Novel dimethylamino modified PDA NPs is applied as cfDNA scavenger.

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The high positively charged modified P displays high binding affinity of cfDNA.

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High positive charge density of cfDNA scavenger endows high efficacy RA therapy.

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Novel biocompatible cfDNA scavenger aims for cfDNA associated diseases therapy.

Hydroxyapatite gradient on poly (vinyl alcohol) hydrogels surface to mimic calcified cartilage zone for cartilage repair

Poly (vinyl alcohol) (PVA) hydrogels are considered promising artificial articular cartilage. However, the weak attachment between PVA hydrogels and subchondral bone limit its application in the biomedical field. In this article, we present a new method to improve the mineralization of PVA hydrogels, and fabricate PVA hydrogels with continuously graded hydroxyapatite coating. The surface of the hydrogels was modified by dopamine self-polymerization and alendronate conjugation subsequently. Based on these, we used simulated body fluids to mineralize the hydrogels to mimic calcified cartilage zone. The modified surface of the PVA hydrogels showed excellent mineralization ability with continuously graded hydroxyapatite (HA). As the main component of human bones, HA can be chemically bonded body tissue on the interface, showing great biological activity. With the content of HA increasing, the cell adhesion ability of the hydrogels was enhanced, which helped the hydrogels integrate tightly with subchondral bone. These results demonstrate that the modified hydrogels could be promising substitutes for articular cartilage.

DNA purification using a novel γ-Fe2O3/PEDOT hybrid nanocomposite

We report the synthesis and characterization of a new hybrid magnetic composite formed by the enveloping of magnetic iron oxide nanoparticles (γ-NP) with chains of the conductive polymer PEDOT, and its use for the efficient separation of DNA molecules from complex biological samples, allowing the high yield separation of a pure and high-quality DNA fraction. The successful formation of the γ-NP/PEDOT composite was confirmed by Fourier transform infrared spectroscopy, scanning electron microscopy, UV visible spectroscopy (UV-Vis), and magnetic hysteresis loop measurements. The nanocomposites showed an excellent capacity of DNA adsorption (Q_e ∼ 248 mg/g) in a model system consisting of salmon sperm DNA. When the γ-NP/PEDOT was used in protocols to extract the DNA from complex samples, the corresponding yield was in the range of 6.4 μg (blood) and 7.3 μg (bacteria), as evaluated quality by UV-Vis, PCR analysis, and electrophoresis assays. We also established that the captured DNA does not need to be detached from the nanocomposite for use as seeding material in PCR amplification experiments. These results and the simplicity of the protocols indicate that the γ-NP/PEDOT composite is a promising DNA absorbent, being competitive with the commercially available magnetic purification kits.

Metal-Doped Polydopamine Nanoparticles for Highly Robust and Efficient DNA Adsorption and Sensing

Controlling DNA adsorption on nanomaterials is crucial for a wide range of applications in analytical and biomedical sciences. Polydopamine (PDA) is a versatile material that can be coated on nearly any surface, and thus adsorbing DNA onto PDA can be a general method for indirect DNA functionalization of surfaces. Polyvalent metal ions were reported to promote DNA adsorption on PDA nanoparticles (NPs), but previous works added the metal ions after the formation of PDA. Herein, we compared the effect of polyvalent metal ions added during the synthesis of PDA NPs (called metal-doped) with the effect of polyvalent metal ions added after the synthesis (metal-adsorbed). A series of metal ions including Ca²⁺, Zn²⁺, Ni²⁺, Fe³⁺, and Gd³⁺ were tested, and Zn²⁺ was studied in detail due to its excellent ability for promoting DNA adsorption. With 100 μM Zn²⁺, metal-doped NPs were ∼30% more efficient than metal-adsorbed NPs for DNA adsorption in buffer attributable to a higher metal loading on the surface of the metal-doped NPs. Metal leaching was negligible from the metal-doped NPs, and they showed a remarkably higher robustness than the metal-adsorbed NPs, resulting in a 20-fold higher DNA extraction efficiency from serum. Based on the desorption studies, a higher adsorption affinity for the metal-doped NPs was confirmed. Finally, the Zn²⁺-doped PDA NPs were used for sensitive DNA detection with a limit of detection of 0.45 nM, and the sensor was highly resistant to nonspecific protein and phosphate displacement.

Spherical Nucleic Acid Mediated Functionalization of Polydopamine-Coated Nanoparticles for Selective DNA Extraction and Detection

Magnetic nanoparticles have been widely used for the separation of biomolecules for biological applications due to the mild and efficient separation process. In previous studies, core-shell magnetic nanoparticles (NPs) were designed for DNA extraction without much sequence specificity. In this work, to achieve highly selective DNA extraction, we designed a core-shell magnetic structure by coating polydopamine (PDA) on Fe₃O₄ NPs. Without divalent metal ions, PDA does not adsorb DNA at neutral pH. The Fe₃O₄@PDA NPs were then functionalized with spherical nucleic acids (SNA) to provide a high density of probe DNA. Fe₃O₄@PDA@SNA was also compared with when a linear SH-DNA was covalently attached to the NPs surface, showing a higher density of the probe SNA than SH-DNA can be loaded on the NPs in a remarkably shorter time. Nonspecific DNA extraction was thoroughly inhibited by both probes. DNA extraction by the Fe₃O₄@PDA@SNA was more effective as well as 5-fold faster than by the Fe₃O₄@PDA@SH-DNA, probably due to the favorable standing conformation of DNA strands in SNA. Moreover, extraction by Fe₃O₄@PDA@SNA showed high robustness in fetal bovine serum, and the same design can be used for selective detection of DNA. Finally, the method was also demonstrated on silica NPs and WS₂ nanosheets for coating with PDA and SNA. Altogether, our findings revealed an interesting and general surface modification strategy using PDA@SNA conjugates for sequence-specific DNA extraction.

Simultaneous Preconcentration and Spectrophotometric Determination of Two Colorants (E110 and E133) in Some Foodstuffs Using a New Mussel-Inspired Adsorbent

BACKGROUND

A naturally occurring material, namely sporopollenin (SP), was subjected to an easy physical surface modification process called a polydopamine coating. The treatment changed the acid-base properties of the surface, so that in the new form the SP surface gained a very attractive character for anionic dyes.

OBJECTIVE

The aim of the study was to develop preconcentration and subsequent spectrophotometric determination methods for two anionic colorants, brilliant blue (BB) and sunset yellow (SY), using polydopamine-coated (PDC) SP.

METHOD

The experiments were carried out in a column system, and the effects of experimental parameters were studied to determine optimal conditions for the quantitative, simultaneous spectrophotometric determination of the dyes.

RESULTS

The dyes could be detected at µg/L levels in their binary mixtures, so the detection limits were found to be 1.5 and 4.3 µg/L in the linear dynamic ranges of 0.0-3.5 and 0-8 µg/mL for BB and SY, respectively. The proposed material and procedure led to quantitative recoveries of between 95 and 100% for the dyes.

CONCLUSIONS

The procedure was applied to real food samples containing BB and SY and both dyes were successfully determined in liquid and solid foodstuffs. The mussel-inspired surface modification is proposed as a useful process to modify the surface of SP.

HIGHLIGHTS

The mussel-inspired polydopamine dip-coating method was adopted to modify the surface of SP for the first time. The PDCSP was successfully used to create a new adsorptive preconcentration method for simultaneous spectrophotometric determination of BB and SY in foodstuffs.

A one step method for isolation of genomic DNA using multi-amino modified magnetic nanoparticles

A simple and efficient approach for the rapid extraction of genomic DNA from blood using various amino-modified magnetic nanoparticles (AMNPs) has been described. The salmon sperm DNA was isolated from aqueous solution based on electrostatic interaction between the positively charged amino-groups of AMNPs and the negatively charged phosphate groups of the DNA. The results of ultraviolet-visible (UV-Vis) spectrometry showed that increasing number of amino groups on the AMNPs surface resulted in an improvement in DNA adsorption efficiency. Several variables including the extraction pH, adsorption time, ionic strength and quantity of AMNPs were optimized to achieve the best extraction efficiency with the proposed method. Acceptable adsorption efficiency of 92% and recovery of 91% were achieved using multi-amino modified MNPs (mAMNPs) with an extraction time of 10 min and an overall processing time of 30 min. The mAMNPs enabled genomic DNA capture from human whole blood, and the resulting mAMNP/DNA complexes could be directly used as templates for PCR amplification without the need for complex and time-consuming DNA elution and purification steps. Our results imply that this method can be used as an effective strategy for genomic DNA extraction and may be extended to other types of biological samples.

Magnetic Polydopamine Modified with Choline-Based Deep Eutectic Solvent for the Magnetic Solid-Phase Extraction of Sulfonylurea Herbicides in Water Samples

A novel magnetic solid-phase extraction technique coupled to ultraperformance liquid chromatography has been developed for separation and preconcentration of four sulfonylurea herbicides (sulfosulfuron, bensulfuron-methyl, pyrazosulfuron-ethyl and halosulfuro-methyl) in aqueous samples. The key point of this method was the application of a novel magnetic nanomaterial that composed of a low eutectic solvent as a shell coated on the magnetic core modified by polydopamine. The extensive active sites outside the low eutectic solvent can effectively adsorb the target herbicide in the extraction process. The obtained magnetic adsorbent was characterized with fourier transform infrared spectrometry, scanning electron microscopy and vibrating sample magnetometer. The influence parameters relevant to this method were optimized. Under the optimum conditions, good linearities could be obtained within the range of 1.0-200 μg L-1 for all analytes, with correlation coefficients ≥0.9908. The limit of detections of the method was between 0.0074 and 0.0100 μg L-1 and the relative standard deviations were 1.1-3.6%. The enrichment factor is 66.6. In the final experiment, the proposed method was successfully applied to the analysis of sulfonylurea herbicides residue in environment and drinking-water samples, and the obtained recoveries were between 70.6% and 109.4%.

Stimuli-responsive polydopamine-based smart materials

This review provides in-depth insight into the structural engineering of PDA-based materials to enhance their responsive feature and the use of them in construction of PDA-based stimuli-responsive smart materials.

Application of stimuli-responsive materials for extraction purposes

Stimuli-responsive materials, frequently designated as "smart/intelligent materials", can modify their structure or properties by either a biological, physical, or chemical stimulus which, if properly controlled, could be used for specific applications. Such materials have been studied and exploited in several fields, like electronics, photonics, controlled drugs administration, imaging and medical diagnosis, among others, as well as in Analytical Chemistry where they have been used as chromatographic stationary phases, as part of sensors and for extraction purposes. This review article pretends to provide an overview of the most recent applications of these materials (mostly polymeric materials) in sample preparation for extraction purposes, as well as to provide a general vision of the current state-of-the-art of this field, their potential use and future applications.

Promoting DNA Adsorption by Acids and Polyvalent Cations: Beyond Charge Screening

Adsorbing DNA oligonucleotides onto nanoparticles is the first step in developing DNA-based biosensors, drug delivery systems, and smart materials. Since DNA is a polyanion, it is repelled by negatively charged nanoparticles, which constitute the majority of commonly used nanomaterials. Adding salt such as NaCl to screen charge repulsion is a standard method of promoting DNA adsorption. However, Na⁺ does not supply additional attractive forces. In addition, adding a high concentration of NaCl can cause the aggregation of nanomaterials. In this feature article, we mainly summarize the methods developed in our laboratory to promote DNA adsorption by lowering the pH and by adding polyvalent metal ions, especially transition-metal ions. Various materials including noble metals (gold, silver, and platinum), 2D materials (graphene oxide, MoS₂, WS₂, and MXene), polydopamine, and several metal oxides are discussed. In general, low pH can protonate DNA bases and nanoparticle surfaces, reducing charge repulsion and even leading to attraction, although DNA folding at low pH can sometimes be detrimental to adsorption. Polyvalent metal ions can bridge additional interactions to achieve otherwise impossible adsorption. On the basis of the current understanding, a few future research directions are proposed to further improve DNA adsorption.

Transition Metal-Mediated DNA Adsorption on Polydopamine Nanoparticles

Polydopamine (PDA) is a widely used universal coating for a broad range of materials. Interfacing PDA with various biomolecules, such as DNA, is critical for applications such as sensing, intracellular delivery, and material fabrication. Because of the negative surface charge of PDA at neutral pH, electrostatic repulsion exists between PDA and DNA. In previous studies, modified DNA or low pH was used to overcome this repulsion for DNA adsorption. More recently, divalent Ca²⁺ was found to bridge DNA and PDA. Herein, we studied four transition metals (Mn²⁺, Co²⁺, Zn²⁺, and Ni²⁺) and compared their efficiencies with Ca²⁺ for promoting DNA adsorption. These transition metals induced a more efficient and tighter DNA binding compared to Ca²⁺. In all these cases, the DNA phosphate backbone played a dominant role in adsorption, although DNA bases might also interact with strong binding metals such as Ni²⁺. Moreover, when the adsorption affinity was stronger, sensing was more selective to complementary DNA. Finally, aging of PDA appeared to be detrimental for DNA adsorption, which could be due to further oxidation of PDA. We showed that using Zn²⁺ or Ni²⁺ could considerably relieve the aging effect, while storing PDA at 4 °C could slow down aging.

Calcium ion assisted fluorescence determination of microRNA-167 using carbon dots-labeled probe DNA and polydopamine-coated Fe3O4 nanoparticles

A selective and sensitive fluorescence biosensor is described for determination of microRNA-167 using fluorescent resonant energy transfer (FRET) strategy. The FRET system comprises carbon dots (CDs, donor) labeled with probe DNA (pDNA) and polydopamine (PDA)-coated Fe₃O₄ nanoparticles (Fe₃O₄@PDA NPs, acceptor). The CDs-pDNA can be absorbed onto the surface of Fe₃O₄@PDA NPs because of the strong π interaction between pDNA and PDA. With the enhanced adsorption ability of Fe₃O₄@PDA NPs by Ca²⁺, the fluorescence intensity of CDs at 445 nm (excitation at 360 nm) is quenched. In presence of microRNA-167, the hybridized complex of CDs-pDNA-microRNA-167 will be released from the surface of Fe₃O₄@PDA NPs due to the weak π interaction of the complex and PDA. This results in the fluorescence recovery of CDs. By application of twice-magnetic separation, the biosensor shows a wide linear range of 0.5-100 nM to microRNA-167 with a 76 pM detection limit. The method was applied to the determination of microRNA-167 in samples of total microRNA extractions from A. thaliana seedlings, and the recoveries ranged from 96.4 to 98.3%.

A multicolorimetric assay for rapid detection of Listeria monocytogenes based on the etching of gold nanorods

Listeria monocytogenes (L. monocytogenes) is one of the most common food-borne pathogens. The authors describe a sensitive and reliable multicolorimetric assay for L. monocytogenes using a sensing system based on TMB²⁺ etching of gold nanorods. Apt-MNP was used as the capture probe, and IgY-BSA-MnO₂ NPs was chosen as an oxidase-like nano-artificial enzyme to oxidize TMB to generate TMB²⁺. Under the optimized conditions, the longitudinal shift of localized surface plasmon resonances had a linear correlation with the L. monocytogenes concentration in the range between 10 to 10⁶ cfu mL^-1. Meanwhile, the sensing system can generate vivid color responses as colorful as a rainbow, and the limit of detection is as low as 10 cfu mL^-1 at a glance. Recoveries ranging from 97.4 to 101.3% are found when analyzing spiked food samples without pre-enrichment. In our perception, it shows promise in rapid instrumental and on-site visual detection of L. monocytogenes.

Rapid and sensitive detection of Staphylococcus aureus assisted by polydopamine modified magnetic nanoparticles

Pathogens cause significant morbidity and mortality to humans. Thus, development of fast and reliable methods for detection and identification of pathogens is urgently needed to increase protection level of public health and ensure the safety of consumers. Herein, a rapid and sensitive method has been developed for Staphylococcus aureus (S. aureus) detection based on the dual role of polydopamine modified magnetic nanoparticles (PDA@Fe₃O₄ NPs) combined with polymerase chain reaction (PCR) and capillary electrophoresis (CE). The core-shell type structure PDA@Fe₃O₄ NPs were prepared, which are spherical, about 152 ± 20 nm in diameter and the PDA shell is about 17.5 ± 1.6 nm. PDA@Fe₃O₄ NPs play a dual role including efficient capture of bacteria and extraction of DNA. In the pH range of 3.0-7.0, the capture efficiency of S. aureus by PDA@Fe₃O₄ NPs was more than 95% in 5 min. The adsorption capacity of the PDA@Fe₃O₄ NPs for S. aureus is 1.2 × 10⁸ cfu mg^-1. The efficient capture and concentration of bacteria from large volumes of samples by PDA@Fe₃O₄ NPs avoids the time-consuming culture-enrichment prior to PCR. Interestingly, PDA@Fe₃O₄ NPs were also found to be efficient adsorbents for extraction of genomic DNA from pathogens based on the electrostatic interaction. The process can be finished in 25 min. The PDA@Fe₃O₄ NPs based solid phase extraction combined with PCR and CE allows for detecting the order of 10² cfu mL^-1S. aureus in tap water and orange juice samples. The whole process takes < 5.5 h. The developed method would provide a promising platform for rapid and sensitive detection of pathogens.