Using rhodamine 6G-modified gold nanoparticles to detect organic mercury species in highly saline solutions

Captivating nano sensors for mercury detection: a promising approach for monitoring of toxic mercury in environmental samples

Mercury, a widespread highly toxic environmental pollutant, poses significant risks to both human health and ecosystems. It commonly infiltrates the food chain, particularly through fish, and water resources via multiple pathways, leading to adverse impacts on human health and the environment. To monitor and keep track of mercury ion levels various methods traditionally have been employed. However, conventional detection techniques are often hindered by limitations. In response to challenges, nano-sensors, capitalizing on the distinctive properties of nanomaterials, emerge as a promising solution. This comprehensive review provides insight into the extensive spectrum of nano-sensor development for mercury detection. It encompasses various types of nanomaterials such as silver, gold, silica, magnetic, quantum dot, carbon dot, and electrochemical variants, elucidating their sensing mechanisms and fabrication. The aim of this review is to offer an in-depth exploration to researchers, technologists, and the scientific community, and understanding of the evolving landscape in nano-sensor development for mercury sensing. Ultimately, this review aims to encourage innovation in the pursuit of efficient and reliable solutions for mercury detection, thereby contributing to advancements in environmental protection and public health.

Colloidal surface-enhanced Raman spectroscopic study of grouper epidermal mucus using acidified sodium sulphate as the aggregating agent

Fish epidermal mucus is an important reservoir of antipathogenic compounds which serves as the first line of the immune defence. Despite its significant role in the physiology and health of fish, detailed profiling of fish epidermal mucus has yet to be explored. Therefore, this study investigates a label-free colloidal surface-enhanced Raman spectroscopic (SERS) method for profiling grouper mucus. Gold nanoparticles were first synthesised using the standard citrate reduction and characterised using ultraviolet-visible spectroscopy, transmission electron microscopy and dynamic light scattering. The influence of acidified sodium sulphate (Na2SO4) at pH 3 as the aggregating agent on the enhancement of the SERS spectrum of different analyte samples including rhodamine 6G (R6G) dye, lysozyme solution and hybrid grouper (Epinephelus fuscoguttatus × Epinephelus lanceolatus) mucus was observed. Based on the results, an optimal Na2SO4 concentration of 1 M was recorded to achieve the highest enhancement of the SERS signal for R6G and grouper mucus, while the optimal concentration for lysozyme was 0.1 M. The results indicated a higher degree of aggregation induced by lysozyme than R6G and grouper mucus. A few overlapping peaks of the SERS spectra of lysozyme and grouper mucus made it possible to confirm the presence of lysozyme as potential biomarkers.

A SYBR Green I-based aptasensor for the label-free, fluorometric, and anti-interference detection of MeHg. Anal Bioanal Chem 2024;416:299-311. [PMID: 37932512 DOI: 10.1007/s00216-023-05018-6] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

Methylmercury (MeHg+) is a common form of organic mercury that is substantially more toxic than inorganic mercury and is more likely to accumulate in organisms through biological enrichment. Therefore, developing a method to enable the specific and rapid detection of MeHg+ in seafood is important and remains challenging to accomplish. Herein, a rapid, label-free fluorescence detection method for MeHg+ determination was developed based on SYBR Green I. The detection system implemented "add and measure" detection mode can be completed in 10 min. Under optimal assay conditions, the detection platform showed a linear relationship with the concentration of MeHg+ within 1-50 nM (Y = 8.573x + 42.89, R2 = 0.9928), with a detection limit of 0.3218 nM. The results obtained for competitive substances, such as inorganic mercury ions and anions, show a high specificity of the method. In addition, this method successfully detected MeHg+ in seawater and marine products, with an accompanying spike recovery rate of 96.45-105.1%.

Detection and imaging of Hg(II) in vivo using glutathione-functionalized gold nanoparticles

The optical and biological properties of functionalized gold nanoparticles (GNPs) have been widely used in sensing applications. GNPs have a strong binding ability to thiol groups. Furthermore, thiols are used to bind functional molecules, which can then be used, for example, to detect metal ions in solution. Herein, we describe 13 nm GNPs functionalized by glutathione (GSH) and conjugated with a rhodamine 6G derivative (Rh6G2), which can be used to detect Hg(II) in cells. The detection of Hg²⁺ ions is based on an ion-catalyzed hydrolysis of the spirolactam ring of Rh6G2, leading to a significant change in the fluorescence of GNPs-GSH-Rh6G2 from an "OFF" to an "ON" state. This strategy is an effective tool to detect Hg²⁺ ions. In cytotoxicity experiments, GNPs-GSH-Rh6G2 could penetrate living cells and detect mercury ions through the fluorescent "ON" form.

Revealing the mechanisms of mercury adsorption on metal-doped kaolinite(001) surfaces by first principles

Natural kaolinite exhibit high affinity for heavy metals while the interaction mechanisms in the presence of heteroatoms remain largely elusive, which are tackled by first principles. In this paper, three common dopants (Mg, Ca, Fe) were employed to construct metal-doped kaolinite(001) (K(001)) surfaces. We found that Mg-doped K(001) was the most stable surface in terms of thermal stability and structural analysis, consistent with the pervasive isomorphic substitution in kaolinite minerals. The interaction of mercury with Mg-doped K(001) surface was investigated in the form of predominant top-site and bridge-site models. The effects of chloride on the interaction were also studied. The results demonstrated that the strongest adsorption occured in the present of dopants and the absence of chloride. The electronic properties revealed a significant charge transfer (up to 1.28 electrons) and chemisorption character at the interfaces when dopants were introduced, which could be ascribed to the overlapping of Hg-5d and O_s-2p (surface O) orbitals in the range of -7.5 eV to +0.5 eV. Additionally, the chloride had a profoundly adverse influence on mercury adsorption due to the upward shift of Hg-6s and Hg-6p orbitals. The studies are beneficial to understand the interaction mechanisms of natural minerals toward environmental pollutants in actual applications.

Detection of trace mercury ions in water with a bovine-serum-albumin-modified Au@SiNWA surface-enhanced-Raman-scattering sensor

Accurate detection of mercury ions (Hg2+) in water is of great importance for environmental protection. Here, a surface-enhanced Raman scattering (SERS) sensor using bovine-serum-albumin-modified gold-silicon nanowire arrays (Au@SiNWAs) is used to detect the ions. The SiNWAs were grown via chemical etching; the addition of modified gold particles on the surface formed Au@SiNWAs to increase the surface activity. The Raman enhancement factor was as large as ∼2.52 × 105, which was also confirmed with finite-difference time-domain simulations. The detection limit for Hg2+ ions in water was as low as ∼10-12 M, which is much lower than that stipulated by the United States Environmental Protection Agency's maximum residue requirements for drinking water. Furthermore, the SERS intensity was linear with the log of the Hg2+ concentration between 1 pM and 100 nM, with a correlation coefficient of 0.992. There was no significant interference when other metal ions were present, which shows the excellent selectivity of the SERS sensor. Unknown Hg2+ concentrations in water can be readily determined in an accurate and reliable manner, with a relative standard deviation of ∼9.21%.

Detection of the chiral drug Ephedrine by resonance Rayleigh scattering based on Ce3+ functionalized gold nanoparticles

Two chiral drugs, ephedrine (EH) and pseudoephedrine (PEH), were commonly used in clinical treatment. Ephedrine (EH) and pseudoephedrine (PEH) could make different changes in resonance Rayleigh scattering spectrum of the detection system which use Ce³⁺ functionalized gold nanoparticles as probe. Therefore, a new method of detecting EH and PEH separately was developed. The RRS spectrum and UV-Vis absorption spectrum of AuNPs-Ce³⁺ detection system was analyzed in order to discuss the mechanism. Under optimal experimental conditions, the linear range of EH and PEH were 20-920 ng/mL and 40-520 ng/mL, respectively. The detection limit were 1.9 ng/mL and 3.8 ng/mL, respectively. Last used for actual testing, this method had obtained good results.

Tributylhexadecylphosphonium Modification Strategy to Construct Gold Nanoprobes for the Detection of Aqueous Cr(III)-Organic Complexes

Most metal probes based on gold nanoparticles (AuNPs) are designed for free metal ions in synthetic waters, and very few are applicable in the detection of metal-organic complexes ubiquitous in real water samples. In this study, we proposed a novel colorimetric nanoprobe strategy for complexed Cr(III) species based on the analyte-induced aggregation of AuNPs, as coated by a cationic surfactant tributylhexadecylphosphonium bromide (THPB) instead of traditional carboxyl modifiers. Such a detection system could be realized via both naked eye and/or UV-vis spectroscopy with detection limits of 8.0 and 0.29 μM, respectively, much lower than its allowable maximum level in industrial effluent as regulated by China EPA (1.5 mg Cr/L, ∼30 μM). The proposed detection system also exhibits high selectivity against various interfering substances including free ions, small organic molecules, and other metal-citrate complexes. The unique hydrolysis and extremely slow decomplexation of Cr(III) are believed to favor the formation of the specific interaction between Cr(III)-citrate and THPB-AuNPs, as verified by X-ray photoelectron spectroscopy characterization, thus endowing the nanoprobe with specific discrimination of the complexed Cr(III) via the aggregation of THPB-AuNPs. Also, the THPB-AuNPs could be stored at room temperature for 30 days and maintain constant detection performance. Moreover, the quantitative detection of Cr(III)-organic complexes with the background of various real water samples agreed well with that based on inductively coupled plasma atomic emission spectrometry, making it an attractive alternative for on-site detection of authentic samples containing Cr(III)-organic species.

Applications of surface functionalized Fe3O4 NPs-based detection methods in food safety

Food safety has always been an issue of great concern to people. The development of rapid, sensitive and specific detection technology of food pollutants is one of the hot issues in food science field. The rapid development of functionalized Fe₃O₄ nanoparticles (NPs) provides unprecedented opportunities and technical support for the innovation of food safety detection. The surface functionalized Fe₃O₄ NPs, which combine superparamagnetic with nanoscale feature, have become an excellent tool for food quality and safety detection. This review highlights the mechanism, principles, and applications of surface functionalized Fe₃O₄ NPs-based detection technique in the agrifood industry. Then the relevant characteristics, functional roles and general mechanisms of nanomaterial-based detection of various endogenous components and exogenous pollutants in foods are discussed in detail. Ultimately, this review is expected to promote the optimization of functionalized Fe₃O₄ NPs and provide direction for the diversity of signal recognition and the sustainability of detection methods.

Discrimination of cysteamine from mercapto amino acids through isoelectric point-mediated surface ligand exchange of β-cyclodextrin-modified gold nanoparticles

A pI-mediated R6G-β-CD@AuNPs system was designed for the first time for the discrimination of CA from GSH/Cys/Hcy in human serum samples.

A novel chromone based colorimetric sensor for highly selective detection of copper ions: Synthesis, optical properties and DFT calculations

In this work, a new chromone based colorimetric sensor (ChrCS) was developed for highly selective detection of copper ions in semi-aqueous media. Evaluation of color and spectral changes displayed by the developed sensor shows that the sensor can be applied to detect copper ions in the presence of other competing metal ions and anions. The developed sensor, which contains biologically active chromone ring, shows excellent selectivity at microlevel for Cu²⁺ with a color change from colorless to yellow. Job's plot based on spectroscopic data showed the complex formation between ChrCS and Cu²⁺ ions has the stoichiometric ratio of 1:1 (ChrCS-Cu²⁺ complex). In addition, the binding constant of the ChrCS to Cu²⁺ was determined using the Benesi-Hildebrand equation. Furthermore, the test papers of the developed ChrCS were successfully prepared and employed to detect different concentration Cu²⁺ (10^-3 M to 10^-7 M) in aqueous solution. Importantly, sensor ChrCS was applied to detect Cu²⁺ ions in real water samples. To better understand the optical character of ChrCS and the effect of metal ion titration, density functional theory (DFT) calculations at the B3LYP/6-31 + G(d,p) level were performed for ChrCS and its complex ChrCS-Cu²⁺. Furthermore, on the basis of the Job's plot analysis DFT calculations, and reversible nature of the developed sensor, the sensing mechanism was demonstrated.

Plasmonic nanoprobes based on the shape transition of Au/Ag core–shell nanorods to dumbbells for sensitive Hg-ion detection

Sensitive plasmonic nanoprobes for the sensitive detection of mercury ions based on a “rod-like to dumbbell or not” morphology transition of the Au/Ag core–shell hybrid nanorods.

Etching-dependent fluorescence quenching of Ag-dielectric-Au three-layered nanoshells: The effect of inner Ag nanosphere

In this report, Ag-dielectric-Au three-layered nanoshells with controlled inner core size were synthesized. The fluorescence emission of the rhodamine 6G (R6G) could be quenched by the three-layered nanoshells distinctly. What's more, the fluorescence quenching efficiency could be further improved by tuning the etching of inner Ag nanosphere. The maximum fluorescence quenching efficiency is obtained when the separate layer just appears between the inner Ag core and the outer Au shell. Whereas the fluorescence quenching efficiency is weakened when no gaps take place around the inner Ag core or the separate layer is too thick and greater than 13nm. The fluorescence quenching properties of the Ag-dielectric-Au three-layered nanoshells with different initial sizes of the Ag nanoparticles are also studied. The maximum fluorescence quenching efficiency is obtained when the three-layered nanoshells are synthesized based on the Ag nanoparticles with 60nm, which is better than others two sizes (42 and 79nm). Thus we believe that the size of initial Ag nanospheres also greatly affects the optimized fluorescence quenching efficiency. These results about fluorescence quenching properties of Ag-dielectric-Au three-layered nanoshells present a potential for design and fabrication of fluorescence nanosensors based on tuning the geometry of the inner core and the separate layer.

Hydride generation coupled with thioglycolic acid coated gold nanoparticles as simple and sensitive headspace colorimetric assay for visual detection of Sb(III)

Antimony (Sb) is a toxic element which causes different health problems including cardiac problems and lung cancer in humans, and its levels in surface water can be noticeably increased to 100 μg/L typically in the proximity of anthropogenic sources. Thus, besides instrumental techniques, it is of great significance to develop a simple, sensitive and selective analytical method for direct analysis of Sb(III) at trace level without the need of any expensive and/or complicated instrumentations and sample preparation processes. Herein, a simple and sensitive headspace colorimetric assay was developed for the detection of Sb(III) by hydride generation coupled with thioglycolic acid functionalized gold nanoparticles (TGA-AuNPs). Sb(III) in the 30 mL sample solution was converted into its volatile form (SbH₃) through hydride generation reaction and headspace extracted into 100 μL chromogenic reagent, which contains methanol as extractant and TGA-AuNPs as nanosensors, leading to aggregation of TGA-AuNPs and therefore a red-to-blue color change. Parameters influencing the chromogenic and hydride generation reactions were optimized. Addition of 300 μM ethylenediamine tetraacetic acid (EDTA) as masking agent largely suppressed the inferences from mercury and arsenic. The proposed method can tolerate at least 10-fold As(III) and 100-fold other metal ions including Hg(II). The detection limits were 6.0 and 1.2 μg/L Sb(III) by naked-eye and UV-Vis spectrometer, respectively, which meet the maximum admissible level in drinking water (6 μg/L) set by the United States Environmental Protection Agency. The feasibility of the proposed method was demonstrated by rapid detection of Sb(III) in river water, lake water, ground water and sea water samples by naked-eye at a spiking level of 6 μg/L Sb(III).

Selectable Ultrasensitive Detection of Hg(2+) with Rhodamine 6G-Modified Nanoporous Gold Optical Sensor

An extremely sensitive fluorescence sensor has been developed for selectively detection of mercury ions based on metallophilic Hg(2+)-Au(+) interactions, which results in an effective release of pre-adsorbed rhodamine 6G (R6G) molecules from the nanoporous gold substrate, associated with a significant decrease of fluorescence intensity. The optical sensor has a detection sensitivity down to 0.6 pM for Hg(2+) and CH3Hg(+) ions, in particular a superior selectivity in a complex aqueous system containing 13 different types of metal ions, meanwhile maintaining a long-term stability after 10 cycles. Such a fluorescence sensor combining multiple advantages therefore present promising potentials in various applications.

Sensitive and selective detection of mercury ions based on papain and 2,6-pyridinedicarboxylic acid functionalized gold nanoparticles

Here we demonstrate the rational design of a sensitive and selective colorimetric method for mercury ion (Hg²⁺) detection by using papain and 2,6-pyridinedicarboxylic acid (PDCA) functionalized gold nanoparticles (AuNPs).

Fluorescence turn-on detection of mercury ions based on the controlled adsorption of a perylene probe onto the gold nanoparticles

A novel fluorescence turn-on strategy based on Au nanoparticles and a perylene probe for the sensing of Hg(2+) ions has been developed. It was observed that a perylene probe could be adsorbed onto the surface of Au NPs through strong electrostatic and hydrophobic interactions. Its fluorescence was efficiently quenched by the Au nanoparticles. However, in the presence of Hg(2+) and NaBH4, Hg(2+) was reduced and an Au/Hg amalgam was formed on the surface of the Au nanoparticles. The perylene probe could hardly be adsorbed and quenched by the Au/Hg amalgam. A turn on fluorescence signal was therefore detected. The assay is quite sensitive, and 5 nM Hg(2+) could be easily detected. It is also very selective, a number of metal ions were tested and no noticeable interference was observed. The assay was also successfully applied for the determination of Hg(2+) in lake water samples. A simple, fast, inexpensive, highly sensitive and selective Hg(2+) sensing strategy is therefore established.

Enhanced in vitro cytotoxicity and cellular uptake of DNA bases functionalized gold nanoparticles in HeLa cell lines

DNA base functionalized AuNPs are internalized to a greater extent with high cellular uptake and enhanced cytotoxicity in HeLa cells.

Nanomaterial-based approaches for the detection and speciation of mercury

Detection and speciation of Hg through the T–Hg–T coordination, Hg–S and/or Hg–Au/Ag interactions based colorimetric or fluorescent changes.

Gold nanoparticle-based fluorescent “turn-on” sensing system for the selective detection of mercury ions in aqueous solution

A simple and straightforward fluorometric assay using dye-adsorbed gold nanoparticles (AuNPs) was used in the highly selective and sensitive detection of mercury ions in aqueous buffer solution.

Sensitive and selective determination of fluvoxamine maleate using a sensitive chemiluminescence system based on the alkaline permanganate-Rhodamine B-gold nanoparticles reaction

A high-yield chemiluminescence (CL) system based on the alkaline permanganate-Rhodamine B reaction was developed for the sensitive determination of fluvoxamine maleate (Flu). Rhodamine B is oxidized by alkaline KMnO4 and a weak CL emission is produced. It was demonstrated that gold nanoparticles greatly enhance this CL emission due to their interaction with Rhodamine B molecules. It is also observed that sodium dodecyl sulfate, an anionic surfactant, can strongly increase this enhancement. In addition, it was demonstrated that a notable decrease in the CL intensity is observed in the presence of Flu. This may be related to Flu oxidation with KMnO4 . There is a linear relationship between the decrease in CL intensity and the Flu concentration over a range of 2-300 µg/L. A new simple, rapid and sensitive CL method was developed for the determination of Flu with a detection limit (3s) of 1.35 µg/L. The proposed method was used for the determination of Flu in pharmaceutical and urine samples.

Preferences of rhodamine coupled (aminoalkyl)-piperazine probes towards Hg(II) ion and their FRET mediated signaling

The metal ion induced absorption and emission signaling pattern of rhodamine coupled bis-(aminopropyl)-piperazine (1-3) and (aminoethyl)-piperazine (4) based probes evaluated in MeCN as well as in an MeCN-H2O binary mixture medium revealed that these probes exhibit optical signaling perturbations to a varying extent in MeCN, however, their complexation induced signaling could be tuned selectively towards Hg(II) in the presence of an aqueous component in the solvent medium where competitive interactions such as metal-probe interactions and hydration of metal ions play the determining factor to induce aqueous promoted Hg(II) selectivity. Attachment of another fluorophore (anthracene and nitrobenzofurazan moieties in 2 and 3 respectively) at the other end of the rhodamine coupled bis-(aminopropyl)-piperazine receptor enabled these probes to facilitate a complexation induced fluorescence resonance energy transfer (FRET) from the excited fluorophore to the ring-opened rhodamine along with contributions through operative PET inhibition and rhodamine delactonization processes. The enhancement in absorption transition of these probes at ~557 nm upon selective Hg(II)-complexation and consequent colourless to pink colour change in the solution imply a chromogenic signaling pattern whereas simultaneous fluorescence amplification and/or FRET initiation lead to fluorogenic signaling to facilitate detection at lower concentration. The Hg(II)-selective photo-physical spectral modulation in the presence of other competitive metal ions, and their reversible dual channel signaling pattern under the action of counter anions or chelating agents such as EDTA or ethylenediamine establish the potential of these probes for highly selective, sensitive and reversible 'OFF-ON-OFF' detection of Hg(II). The complexation induced optical signaling pattern of probes with a propyl-linker in their receptor (1-3) in comparison with that of 4 consisting of an ethyl-spacer indicate that signaling probe design with a substituted 'aminoalkyl-lactonized-rhodamine' subunit preferentially exhibit Hg(II) selective and sensitive dual mode signaling in an organic-aqueous mixture medium irrespective of carbon-length of the flexible alkyl spacer.

Ultrasensitive detection of toxic cations through changes in the tunnelling current across films of striped nanoparticles

Although multiple methods have been developed to detect metal cations, only a few offer sensitivities below 1 pM, and many require complicated procedures and sophisticated equipment. Here, we describe a class of simple solid-state sensors for the ultrasensitive detection of heavy-metal cations (notably, an unprecedented attomolar limit for the detection of CH(3)Hg(+) in both standardized solutions and environmental samples) through changes in the tunnelling current across films of nanoparticles (NPs) protected with striped monolayers of organic ligands. The sensors are also highly selective because of the ligand-shell organization of the NPs. On binding of metal cations, the electronic structure of the molecular bridges between proximal NPs changes, the tunnelling current increases and highly conductive paths ultimately percolate the entire film. The nanoscale heterogeneity of the structure of the film broadens the range of the cation-binding constants, which leads to wide sensitivity ranges (remarkably, over 18 orders of magnitude in CH(3)Hg(+) concentration).

Selective detection of iodide and cyanide anions using gold-nanoparticle-based fluorescent probes

We developed two simple, rapid, and cost-effective fluorescent nanosensors, both featuring bovine serum albumin labeled with fluorescein isothiocyanate (FITC))-capped gold nanoparticles (FITC-BSA-Au NPs), for the selective sensing of cyanide (CN(-)) and iodine (I(-)) ions in high-salinity solutions and edible salt samples. During the preparation of FITC-BSA-Au NP probes, when AuNPs were introduced to the mixture containing FITC and BSA, the unconjugated FITC and FITC-labeled BSA (FITC-BSA) adsorbed to the particles' surfaces. These probes operated on a basic principle that I(-) and CN(-) deposited on the surfaces of the Au NPs or the etching of Au NPs induced the release of FITC molecules or FITC-BSA into the solution, and thus restored the florescence of FITC. We employed FITC-BSA to protect the Au NPs from significant aggregation in high-salinity solutions. In the presence of masking agents such as S(2)O(8)(2-)/Pb(2+), FITC-BSA-Au NPs facilitated the selective detection of CN(-) (by at least 150-fold in comparison with other anions). We also demonstrated that the FITC-BSA-Au NPs in the presence of H(2)O(2) could selectively detect I(-) down to 50 nM. Taking advantages of their high stability and selectivity, we employed our FITC-BSA-Au NP-based probes for the detection of CN(-) and I(-) in water samples (pond water, tap water, and seawater) and detection of I(-) in edible salt samples, respectively. This simple, rapid, and cost-effective sensing system appears to demonstrate immense practical potential for the detection of anions in real samples.

Antibacterial activities of tellurium nanomaterials

We prepared four differently shaped Te nanomaterials (NMs) as antibacterial reagents against Escherichia coli. By controlling the concentrations of hydrazine (N(2)H(4)) as reducing agent, NaCl, and temperature, we prepared Te nanowires, nanopencils, nanorices, and nanocubes. These four Te NMs resulted in a live/dead ratio of E. coli cells of less than 0.1, which is smaller than that of Ag nanoparticles. The order of antibacterial activity against E. coli is nanocubes ≈ nanorices > nanopencils ≈ nanowires. This is in good agreement with the concentration order of tellurite (TeO(3)(2-)) ions released from Te NMs in E. coli cells, revealing that TeO(3)(2-) ions account for the antibacterial activity of the four Te NMs. We found that spherical Te nanoparticles (32 nm in diameter) with TeO(3)(2-) ions were formed in the E. coli cells. Compared to Ag nanoparticles that are commonly used as antibacterial reagents, Te NMs have higher antibacterial activity and lower toxicity. Thus, Te NMs hold great practical potential as a new and efficient antibacterial agent.