Nanotechnology Methods Research Articles

Valorization of Vaccinium corymbosum waste from the extraction of bioactive compounds: Nanoparticles synthesis and applications

This study presents the biogenic synthesis of silver nanoparticles (Ag NPs), zinc oxide nanoparticles (ZnO NPs), and iron oxide nanoparticles (FeO NPs) using V. corymbosum extracts as a reducing agent, varying the ultrasound extraction times (t1 = 20 min, t2 = 15 min, t3 = 10 min). Ag NPs exhibited an antimicrobial efficiency of 98.2 %, being most effective with t2 due to their smaller size. ZnO NPs demonstrated the best thermal stability with t1, and FeO NPs showed optimal magnetic properties with t3. The reduction of methylene blue by ZnO and FeO NPs was significant, with ZnO-t1 and FeO-t3 standing out. The nanoparticles exhibited distinctive optical characteristics and adequate morphology, depending on the extraction time. This work highlights the feasibility of using natural extracts to synthesize nanoparticles, promoting sustainable methods in nanoscience and nanotechnology. Future research should focus on the functionalization of NPs to expand their applications.

Possible Combinatorial Utilization of Phytochemicals and Extracellular Vesicles for Wound Healing and Regeneration.

Organ and tissue damage can result from injury and disease. How to facilitate regeneration from damage has been a topic for centuries, and still, we are trying to find agents to use for treatments. Two groups of biological substances are known to facilitate wound healing. Phytochemicals with bioactive properties form one group. Many phytochemicals have anti-inflammatory effects and enhance wound healing. Recent studies have described their effects at the gene and protein expression levels, highlighting the receptors and signaling pathways involved. The extremely large number of phytochemicals and the multiple types of receptors they activate suggest a broad range of applicability for their clinical use. The hydrophobic nature of many phytochemicals and the difficulty with chemical stabilization have been a problem. Recent developments in biotechnology and nanotechnology methods are enabling researchers to overcome these problems. The other group of biological substances is extracellular vesicles (EVs), which are now known to have important biological functions, including the improvement of wound healing. The proteins and nanoparticles contained in mammalian EVs as well as the specificity of the targets of microRNAs included in the EVs are becoming clear. Plant-derived EVs have been found to contain phytochemicals. The overlap in the wound-healing capabilities of both phytochemicals and EVs and the differences in their nature suggest the possibility of a combinatorial use of the two groups, which may enhance their effects.

A comprehensive review: Advancements in nanomaterials on the risk prevention, detection, and elimination of mycotoxin contamination

Mycotoxins are poison that filamentous fungi generate under specific conditions. Mycotoxins in food and feed have a detrimental effect on both human and animal health, resulting in significant financial losses for agriculture sector. Despite the continuing advancement of traditional approaches, modern research trends favor novel alternatives. Therefore, it is crucial to prevent mycotoxin contamination, which has raised concerns around the globe. Recent advancements in the management of mycotoxin contamination have been possible by the application of promising new nanomaterials. Mycotoxins have negative impacts on human health, but nanotechnology methods appear to be viable, efficient, and affordable solutions. This review elucidates information on the incidence and toxicology of mycotoxins. Nanotechnology’s potential for removal of mycotoxins is mentioned briefly. Then, attention is directed on using newly developed nanomaterials to regulate mycotoxin contamination, such as testing, production, inhibition, adsorption, and removal of mycotoxins. The issues regarding the toxicity, incidence, and management of mycotoxins are tentatively presented along with potential prospects for using nanotechnology to remove mycotoxins from food and feed.

A Review on Synthesis Methods of Materials Science and Nanotechnology

A Review on Synthesis Methods of Materials Science and Nanotechnology

Optimizing Binding among Bimolecular Tethered Complexes.

Tethered motion is ubiquitous in nature, offering controlled movement and spatial constraints to otherwise chaotic systems. The enhanced functionality and practical utility of tethers has been exploited in biotechnology, catalyzing the design of novel biosensors and molecular assembly techniques. While notable technological advances incorporating tethered motifs have been made, a theoretical gap persists within the paradigm, hindering a comprehensive understanding of tethered-based technologies. In this work, we focus on the characterization of the binding kinetics of two tethered molecules functionalized to a hard surface. Using a mean-field approximation, the binding time of such bimolecular system is determined analytically. Furthermore, estimates of the grafting site separation and polymer lengths which expedite binding are provided. These estimates, along with the analytical theories and frameworks established here, have the potential to improve efficacy in self-assembly methods in DNA nanotechnology and can be extended to more biologically specific endeavors including targeted drug-delivery and molecular sensing.

Revitalizing Cancer Treatment: Exploring the Role of Drug Repurposing.

Cancer persists as a global challenge necessitating continual innovation in treatment strategies. Despite significant advancements in comprehending the disease, cancer remains a leading cause of mortality worldwide, exerting substantial economic burdens on healthcare systems and societies. The emergence of drug resistance further complicates therapeutic efficacy, underscoring the urgent need for alternative approaches. Drug repurposing, characterized by the utilization of existing drugs for novel clinical applications, emerges as a promising avenue for addressing these challenges. Repurposed drugs, comprising FDA-approved (in other disease indications), generic, off-patent, and failed medications, offer distinct advantages including established safety profiles, cost-effectiveness, and expedited development timelines compared to novel drug discovery processes. Various methodologies, such as knowledge-based analyses, drug-centric strategies, and computational approaches, play pivotal roles in identifying potential candidates for repurposing. However, despite the promise of repurposed drugs, drug repositioning confronts formidable obstacles. Patenting issues, financial constraints associated with conducting extensive clinical trials, and the necessity for combination therapies to overcome the limitations of monotherapy pose significant challenges. This review provides an in-depth exploration of drug repurposing, covering a diverse array of approaches including experimental, re-engineering protein, nanotechnology, and computational methods. Each of these avenues presents distinct opportunities and obstacles in the pursuit of identifying novel clinical uses for established drugs. By examining the multifaceted landscape of drug repurposing, this review aims to offer comprehensive insights into its potential to transform cancer therapeutics.

Quantum dots: modern methods of synthesis and optical properties

Quantum dots are the most exciting representatives of nanomaterials. They are synthesized using advanced methods of nanotechnology pertaining to both inorganic and organic chemistry. Quantum dots possess unique physical and chemical properties; therefore, they are used in very different fields of physics, chemistry, biology, engineering and medicine. It is not surprising that the Nobel Prize in chemistry in 2023 was given for discovery and synthesis of quantum dots. This review addresses modern methods for the synthesis of quantum dots and their optical properties and practical applications. In the beginning, a short insight into the history of quantum dots is given. Many gifted scientists, including chemists and physicists, were engaged in these studies. The synthesis of quantum dots in solid and liquid matrices is described in detail. Quantum dots are well-known owing to their unique optical properties; that is why the attention in the review is focused on the quantum-size effect. The causes for fascinating blinking of quantum dots and techniques for observation of a single quantum dot are considered. The last part of the review describes mportant applications of quantum dots in biology, medicine and quantum technologies.<br> The bibliography includes 772 references.

Surface-Enhanced Raman Scattering: 50 Years of Development and Its Role in Nanobiotechnology

The review covers the main stages in the development and achievements of surface-enhanced Raman scattering (SERS), the fiftieth anniversary of which we celebrate in 2024. Modern theoretical views on the SERS effect, a retrospective of the substrates and materials used, and expansion of the range of studied compounds and objects are presented. Using the example of the most important and interesting applications of SERS spectroscopy, the achievements of the last decade are considered; its role in modern surface chemistry, nanotechnology, bioanalysis and bioimaging methods is shown.

Artificial Intelligence Integration with Nanotechnology: A New Frontier for Sustainable and Precision Agriculture

Abstract: Addressing the challenges posed by climate change, surging population, rival demands on land for renewable fuel manufacturing, and adverse soil conditions is crucial for ensuring global food security. Achieving sustainable solutions necessitates the integration of multidisciplinary knowledge, such as materials technology and informatics. The convergence of precision agriculture with nanotechnology and artificial intelligence (AI) offers promising prospects for sustainable food production. Through real-time responsiveness to crop growth using advanced technologies, such as nanotechnology and AI, farmers can optimize resource allocation and make informed decisions. Newer opportunities for sustainable food production arise through the integration of precision agriculture, nanotechnology, and artificial intelligence. This convergence enables farmers to dynamically respond to crop growth variations using advanced techniques. By combining nanotechnology and informatics methods with existing models for nutrient cycling and crop productivity, it becomes possible to enhance critical aspects, such as precision targeting, efficient absorption, effective distribution, optimized nutrient assimilation, and long-term effects on soil microbial communities. This integration offers significant potential for improving agriculture and addressing sustainability challenges in food production. Ultimately, this synergy allows for the development of nanoscale agrochemicals that offer a balance between safety and functionality, ensuring optimal performance in agricultural systems.

Nano Strategies for Artemisinin Derivatives to Enhance Reverse Efficiency of Multidrug Resistance in Breast Cancer.

Artemisinin (ART) has been found to exert anti-tumor activity by regulating the cell cycle, inducing apoptosis, inhibiting angiogenesis and tumor invasion and metastasis. Its derivatives (ARTs) can regulate the expression of drug-resistant proteins and reverse the multidrug resistance (MDR) of tumor cells by inhibiting intracellular drug efflux, inducing apoptosis and autophagy of tumor cells, thus enhancing the sensitivity of tumor cells to chemotherapy and radiotherapy. Recent studies have shown that nanodrugs play an important role in the diagnosis and treatment of cancer, which can effectively solve the shortcomings of poor hydrophilicity and low bioavailability of ARTs in the human body, prolong the in vivo circulation time, improve the targeting of drugs (including tumor tissues or specific organelles), and control the release of drugs in target tissues, thereby reducing the side effect. This review systematically summarized the latest research progress of nano-strategies of ARTs to enhance the efficiency of MDR reversal in breast cancer (BC) from the following two aspects: (1) Chemicals encapsulated in nanomaterials based on innovative anti-proliferation mechanism: non-ABC transporter receptor candidate related to ferroptosis (dihydroartemisinin/DHA analogs). (2) Combination therapy strategy of nanomedicine (drug-drug combination therapy, drug-gene combination, and chemical-physical therapy). Self-assembled nano-delivery systems enhance therapeutic efficacy through increased drug loading, rapid reactive release, optimized delivery sequence, and realization of cascade-increasing effects. New nanotechnology methods must be designed for specific delivery routines to achieve targeting administration and overcome MDR without affecting normal cells. The significance of this review is to expect that ART and ARTs can be widely used in clinical practice. In the future, nanotechnology can help people to treat multidrug resistance of breast cancer more accurately and efficiently.

Nanoscience and -technology in secondary education: A systematic literature review

This article provides a review of empirical research into nanoscience and -technology (NST) education at the secondary school level with regards to (a) teaching strategies or laboratory experiments implemented and evaluated for their impact on student learning about nanotechnology aspects, (b) concept inventories and methods used to assess students’ conceptual understanding, as well as (c) students’ conceptions and learning difficulties. A database search was used to identify corresponding studies published over the last decade (2012-2021) of which eleven were included in the synthesis for further analyses after screening for eligibility. The analysis revealed that learning difficulties regarding NST topics such as the differentiation between size and scale, the surface area to volume ratio or size dependent properties are widely prevalent among learners according to the current state of the literature. While our analysis identified emerging perspectives for future research with regards to the development of psychometrically characterized concept inventories in particular and empirical investigations into students’ learning progressions on nanoscience concepts in more general, a huge effort has already been put into the development of teaching concepts or laboratory experiments suitable for secondary school classrooms.

REPRODUCTIVE ABILITY OF BEE QUEENS AND PRODUCTIVITY OF BEES FEEDING MAGNESIUM CITRATES

The research was conducted on two groups of bees families-analogs, three families in each group. Bees of the control (I) group were fed with 60% of sugar syrup (s.s) in the amount of 2 l / family / week. Bees of the II (experimental) group with 2 l of s.s. received 4 mcg/ml of Mg in the form of citrate obtained by the nanotechnology method from LLC Nanomaterials and Nanotechnologies, Kyiv. Indicators of bee queens egg laying intensity was determined by the method of counting the amount of number of sealed brood. Samples of bee body tissues and honey were investigated, in which the content of individual heavy metals (Fe, Сu, Zn) was determined on an atomic absorption spectrophotometer SF-115 PC, as well as the qualitative indicators of honey. According to the results of the researches during the entire research period bee queens of the control (1st), 2nd research groups oviposited 47783 and 49721 eggs, respectively. Probable inter group differences of egg laying were marked at feeding bee families pure sugar syrup and with adding to it citrates Mg. Amount of oviposited eggs in the 2nd research groups during the research period was higher accordingly by 1,4 % in comparison to the control group. Research results specify on expediency of application of citrates of Mg for stimulation of vital functions of bee colony and increasing the reproductive ability of queen bees during their intensive egg laying
 Researches have shown changes in the correlation of trace elements in body tissues of bees with an increase in the content of Fe, Cu and a decrease in the concentration Zn. Feeding of bees with Mg citrates caused an increase parameters of honey, as evidenced by the probable increase in diastasis number and proline content in honey of bees of the experimental groups. Organoleptic characteristics of honey when feeding bees with Mg citrates did not change significantly in the experimental groups compared with the control and met the current standards of DSTU 4497: 2005. It is concluded that feeding bees with the addition to sugar syrup Mg nanotechnological citrates stimulates the intensity of egg laying by queen bees, mineral exchange in their organism and optimizes the content of trace elements and quality indicators in products.

Broadband optical vortex beam generation using flat-surface nanostructured gradient index vortex phase masks

We developed a new kind of compact flat-surface nanostructured gradient index vortex phase mask, for the effective generation of optical vortex beams in broadband infrared wavelength range. A low-cost nanotechnological material method was employed for this work. The binary structure component consists of 17,557 nano-sized rods made of two lead–bismuth–gallium silicate glasses which were developed in-house. Those small rods are spatially arranged in such a way that, according to effective medium theory, the refractive index of this internal structure is constant in the radial direction and linearly changes following azimuthal angle. Numerical results demonstrated that a nanostructured vortex phase mask with a thickness of 19 μm can convert Gaussian beams into fundamental optical vortices over 290 nm wavelength bandwidth from 1275 to 1565 nm. This has been confirmed in experiments using three diode laser sources operating at 1310, 1550, and 1565 nm. The generation of vortex beams is verified through their uniform doughnut-like intensity distributions, clear astigmatic transformation patterns, and spiral as well as fork-like interferograms. This new flat-surface component can be directly mounted to an optical fiber tip for simplifying vortex generator systems as well as easier manipulation of the generated OVB in three-dimensional space.

Nanotechnologies for environmental remediation and their ecotoxicological impacts.

Environmental nanoremediation is an emerging technology that aims to rapidly and efficiently remove contaminants from the polluted sites using engineered nanomaterials (ENMs). Inorganic nanoparticles which are generally metallic, silica-based, carbon-based, or polymeric in nature serve to remediate through chemical reactions, filtration, or adsorption. Their greater surface area per unit mass and high reactivity enable them to treat groundwater, wastewater, oilfields, and toxic industrial contaminants. Despite the growing interest in nanotechnological solutions for bioremediation, the environmental and human hazard associated with their use is raising concerns globally. Nanoremediation techniques when compared to conventional remediation solutions show increased effectivity in terms of cost and time; however, the main challenge is the ability of ENMs to remove contaminants from different environmental mediums by safeguarding the ecosystem. ENMs improving the accretion of the pollutant and increasing their bioavailability should be rectified along with the vigilant management of their transfer to the upper levels of the food chain which subsequently causes biomagnification. The ecosystem-centered approach will help monitor the ecotoxicological impacts of nanoremediation considering the safety, sustainability, and proper disposal of ENMs. The environment and human health risk assessment of each novel engineered nanomaterial along with the regulation of life cycle assessment (LCA) tools of ENMs for nanoremediation can help investigate the possible environmental hazard. This review focuses on the currently available nanotechnological methods used for environmental remediation and their potential toxicological impacts on the ecosystem.

Effect of Surface, Subsurface Drip Irrigation and Nanotechnology Perfusion Methods on Irrigation Efficiency and Saved Water Ratio in Plowed and Unplowed Soils

A field experiment was conducted in Al-Habbaniyah district / Al- Khalidiya district / Al-Anbar governorate, western Iraq, in Silty loam soil during the fall season 2020, to study the effect of surface and subsurface drip irrigation and perfusion irrigation with nanotechnology on irrigation efficiency, the amount of added water and the percentage of water saving in plowed and non-plowed soils. Perfusion was done when 50% of the prepared water was depleted. The treatments were distributed according to the Nested-Factorial Experiments Design (NFED) with three replications. The yellow corn, cultivar Euphrates, was planted on 2020/15/7. The American evaporation basin, type A, was used in the timing of irrigation. According to the rate of water consumption in terms of the equivalent depth of the added water, the total yield and the efficiency of water use. The results of the study showed the uniqueness of the drip irrigation system by giving the highest irrigation efficiency of 100% for the two plowing modes, while the surface drip irrigation system gave an irrigation efficiency of 83% in the uncultivated soil, and the lowest amount of water was added during the season, which amounted to 5215 m3. ha-1 in the nano-technology perfusion irrigation method with a savings rate of 29.97 and 13.19% compared with the surface drip irrigation method, while it reached 15.23 and 8% compared with the subsurface drip irrigation method in the plowed and non-plowed soils sequentially, the highest average total production was 11.7 tons of grain per hectare for the tillage and no-till treatment, and it decreased significantly to 7.5 tons ha-1 in the surface drip irrigation method, while the tillage treatments did not record significant differences in the short-term grain yield.

Вплив різних доз цитратів наночастинок йоду, селену, сірки на активність гіпофіз-тиреоїдної системи і обмін речовин у щурів

The use of organic compounds of iodine, selenium, and sulfur is limited due to the complexity of their chemical synthesis. The latest nanotechnological methods of obtaining organic compounds I, Se, S provide the opportunity to replace their mineral and organic salts with citrate mixtures synthesized on the basis of nanotechnology. This provides a perspective for the use of nanocomposition citrates I, Se, S in biology, medicine and animal husbandry. The research aimed to determine the effect of different doses of I, Se, S nanoparticles citrates on the activity of the pituitary-thyroid system and metabolism in the blood of male rats. The animals of the experimental groups received daily with water a mixture of I, Se, S in experimentally determined ratios (3:1:5) by mass of elements and doses, according to the concentration of iodine in μg/kg of body weight: 1–2.4 (low); 2–24 (average); 3–240 (high); 4–2400 (toxic). Blood for laboratory studies was taken on the 40th day of drinking I, Se, S after the decapitation of animals. The content of thyroid-stimulating hormone (TSH), triiodothyronine (T3), thyroxine (T4), thyroglobulin (Tg), antibodies to thyroglobulin (Anti-Tg) and thyroperoxidase (Anti-TPO), as well as hematological and biochemical indicators characterizing the activity of physiological systems of the body, were determined in the blood. Activation of the body’s immune system was established with an increase in the number of monocytes in the blood of rats of the experimental groups under the influence of all applied doses of I, Se, S, and platelets – only in doses of 2.4, 24, and 240 μg of iodine. The effect of the toxic dose led to an increase in the number of leukocytes in the blood of animals of the 4 group, with a decrease in platelets. A probable decrease in the relative level of lymphocytes and an increase in monocytes in the blood of rats in the experimental groups characterized changes in the leukogram of blood. The applied doses of I, Se, S stimulated the synthesis of cholesterol and the influx of calcium into the peripheral blood of rats of the experimental groups. Still, they inhibited the activity of alkaline phosphatase and the creatinine content. The toxic effect of I, Se, S nanoparticles citrates in a dose of 2400 μg of I led to a significant increase in the activity of aminotransferases and a decrease in alkaline phosphatase, phosphorus level, and body weight in rats. The effect of high and toxic doses of I, Se, S nanoparticles citrates on the functioning of the pituitary-thyroid system led to a decrease in the concentration of TSH, T3, T4, and Tg against the background of an increase in Anti-TPO and Anti-Tg in the blood serum of rats of experimental groups.

Intranasal delivery of stem cells labeled by nanoparticles in neurodegenerative disorders: Challenges and opportunities.

Neurodegenerative disorders occur through progressive loss of function or structure of neurons, with loss of sensation and cognition values. The lack of successful therapeutic approaches to solve neurologic disorders causes physical disability and paralysis and has a significant socioeconomic impact on patients. In recent years, nanocarriers and stem cells have attracted tremendous attention as a reliable approach to treating neurodegenerative disorders. In this regard, nanoparticle-based labeling combined with imaging technologies has enabled researchers to survey transplanted stem cells and fully understand their fate by monitoring their survival, migration, and differentiation. For the practical implementation of stem cell therapies in the clinical setting, it is necessary to accurately label and follow stem cells after administration. Several approaches to labeling and tracking stem cells using nanotechnology have been proposed as potential treatment strategies for neurological diseases. Considering the limitations of intravenous or direct stem cell administration, intranasal delivery of nanoparticle-labeled stem cells in neurological disorders is a new method of delivering stem cells to the central nervous system (CNS). This review describes the challenges and limitations of stem cell-based nanotechnology methods for labeling/tracking, intranasal delivery of cells, and cell fate regulation as theragnostic labeling. This article is categorized under: Therapeutic Approaches and Drug Discovery > Nanomedicine for Neurological Disease.

The current state of electrospinning technology and its prospects for the future

With the development of nanotechnology and modern research methods, the use of biodegradable polymer materials in various sectors of human life is of interest not only to the scientific world community, but also i a way to solve one of the global problems related to resource conservation and environmental protection. Polyvinyl alcohol-based materials have been widely used in various fields due to their biological and physical properties, such as biocompatibility, biodegradability, antimicrobial ability, non-toxicity and the ability to easily form a film. One of the methods of obtaining polymer films that exist today is electrospinning, the advantages of which are the relative simplicity of the technological process and the possibility of obtaining continuous nanofibers from both synthetic and natural polymers. In this work, the influence of various process parameters on the formation of nanofiber mats from a biodegradable synthetic polymer by electrospinning was studied. The technology of wet spinning, melt spinning and dry spinning is discussed. A number of experimental studies have been carried out to identify optimal modes of obtaining nanofibers from polyvinyl alcohol with the most homogeneous structure without the formation of defects.

Refractive Properties of Conjugated Organic Materials Doped with Fullerenes and Other Carbon-Based Nano-Objects.

Due to the high demand for optoelectronics for use in new materials and processes, as well as the search for their modeling properties, the expansion of the functionality of modified materials using nanotechnology methods is relevant and timely. In the current paper, a specific nanotechnology approach is shown to increase the refractive and photoconductive parameters of the organic conjugated materials. The sensitization process, along with laser treatment, are presented in order to improve the basic physical-chemical properties of laser, solar energy, and general photonics materials. Effective nanoparticles, such as fullerenes, shungites, reduced graphene oxides, carbon nanotubes, etc., are used in order to obtain the bathochromic shift, increase the laser-induced change in the refractive index, and amplify the charge carrier mobility of the model matrix organics sensitized with these nanoparticles. The four-wave mixing technique is applied to test the main refractive characteristics of the studied materials. Volt-current measurements are used to estimate the increased charge carrier mobility. The areas of application for the modified nanostructured plastic matrixes are discussed and extended, while also taking into account the surface relief.

Comparative evaluation of the biological activity of a complex of ultrafine particles based on copper and zinc in the bioluminescence inhibition test

Modern methods of nanotechnology make it possible to transform macro- and micro-objects into ultrafine particles (UFP), while the process itself is accompanied by a change in the chemical and physical properties of the base material. Naturally, the biological activity of complexes of UHF microelements will differ from that of individual UHFs. In this regard, the purpose of our study was to study the dependence of the biological activity of the UHF copper + zinc complex on their physicochemical characteristics.Analysis of the data obtained allowed us to state that sample «C» had a high level of toxicity of the studied samples of UHF against the strain E. coli E. coli K12 TG1 pF1. Further, according to the observed effect, sample A was noted, while the effect of exposure to the studied UHF was dose- and time-dependent.As a result, the data obtained on the basis of the reaction of strains E. coli pSoxS::lux and E. coli pKatG-lux, the induction of luminescence of which is associated with the development of oxidative stress in cells, on the action of the studied samples of the copper+ zinc complex, made it possible to form a sample with the maximum effect «C». Further, according to the degree of development of the toxic effect, samples «B» and «A» decreased in the series. At the same time, the interpretation of the results of the response of the E. coli pRecA -lux strain, the induction of which is caused by DNA damage in cells, forms a different series of specificity: the greatest effect on bacterial cells among the studied substances, sample B was rendered, while the maximum luminescence amplitude was 70 units at 0.002 M, and the sensitivity threshold was at a concentration of 0.0004 M.Based on the data obtained, in our study, the degree of biological activity of UHF for a living system was determined by a combination of a number of physicochemical parameters: shape, phase composition, and specific surface area