nanomaterials Review Microbial Nanotechnology: Challenges and Prospects for Green Biocatalytic Synthesis of Nanoscale Materials for Sensoristic and Biomedical Applications Gerardo Grasso *, Daniela Zane and Roberto Dragone Consiglio Nazionale delle Ricerche—Istituto per lo Studio dei Materiali Nanostrutturati c/o Dipartimento di Chimica, ‘Sapienza’ Università di Roma, P. le Aldo Moro 5, 00185 Roma, Italy; [email protected] (D.Z.); [email protected] (R.D.) * Correspondence: [email protected]; Tel.: +39-064-991-3380 Received: 19 November 2019; Accepted: 13 December 2019; Published: 18 December 2019 Abstract: Nanomaterials are increasingly being used in new products and devices with a great impact on different fields from sensoristics to biomedicine. Biosynthesis of nanomaterials by microorganisms is recently attracting interest as a new, exciting approach towards the development of ‘greener’ nanomanufacturing compared to traditional chemical and physical approaches. This review provides an insight about microbial biosynthesis of nanomaterials by bacteria, yeast, molds, and microalgae for the manufacturing of sensoristic devices and therapeutic/diagnostic applications. The last ten-year literature was selected, focusing on scientific works where aspects like biosynthesis features, characterization, and applications have been described. The knowledge, challenges, and potentiality of microbial-mediated biosynthesis was also described. Bacteria and microalgae are the main microorganism used for nanobiosynthesis, principally for biomedical applications. Some bacteria and microalgae have showed the ability to synthetize unique nanostructures: bacterial nanocellulose, exopolysaccharides, bacterial nanowires, and biomineralized nanoscale materials (magnetosomes, frustules, and coccoliths). Yeasts and molds are characterized by extracellular synthesis, advantageous for possible reuse of cell cultures and reduced purification processes of nanomaterials. The intrinsic variability of the microbiological systems requires a greater protocols standardization to obtain nanomaterials with increasingly uniform and reproducible chemical-physical characteristics. A deeper knowledge about biosynthetic pathways and the opportunities from genetic engineering are stimulating the research towards a breakthrough development of microbial-based nanosynthesis for the future scaling-up and possible industrial exploitation of these promising ‘nanofactories’. Keywords: applied microbiology; white biotechnology; green chemistry; nanostructured materials; diatom nanotechnology; sensoristic devices; drug delivery; theranostics 1. Introduction During the period of 2016–2022 the global nanomaterials market is expected to grow with a compound annual growth rate of about 20% or more [1]. One of the major challenges for the global advancement of nanomaterials market is the environmental sustainability of nanomanufacturing processes. Indeed, traditional top-down or bottom-up chemical and physical nanomanufacturing approaches have a greater energy-intensity compared to manufacturing processes of bulk materials. Further, they are often characterized by low process yields (using acidic/basic chemicals and organic solvents), generation of greenhouse gases, and they require specific facilities, operative conditions (e.g., from moderate to high vacuum), and high purity levels of starting materials [2–4]. The principles Nanomaterials 2020, 10, 11; doi:10.3390/nano10010011 www.mdpi.com/journal/nanomaterials Nanomaterials 2020, 10, 11x FOR PEER REVIEW 22 of of 22 20 principles of green chemistry (“the invention, design and application of chemical products and processesof green chemistry to reduce (“the or to invention, eliminate design the use and and application generation of of chemical hazardous products substances”) and processes combined to reduce with whiteor to eliminate biotechnology the use (“biotechnology and generation of that hazardous uses living substances”) cells—yeasts, combined molds, with bacteria, white biotechnology plants, and (“biotechnologyenzymes to synthesize that uses products living at cells—yeasts, industrial scale” molds,) can bacteria,really contribute plants, andto the enzymes development to synthesize of more productssustainable at industrialindustrial scale”) processes can really [5], contributealso for tonanomanufacturing. the development of moreThe sustainablemicrobial-mediated industrial processesbiosynthesis [5], of also nanomaterials for nanomanufacturing. is a promising The biotechnological-based microbial-mediated biosynthesis nanomanufacturing of nanomaterials process that is a promisingrepresents a biotechnological-based ‘green’ alternative approach nanomanufacturing to physical and process chemical that strategies represents of ananosynthesis ‘green’ alternative [6,7]. Theapproach microbial-mediated to physical and biosynthesis chemical strategiesof metallic of (also nanosynthesis as alloys), [6non-metallic,,7]. The microbial-mediated or metal oxides nanoparticlesbiosynthesis ofhave metallic been re (alsoported as for alloys), many non-metallic,microbial strains or of metal bacteria, oxides yeast, nanoparticles molds, and microalgae have been [8]reported (Figure for 1). many microbial strains of bacteria, yeast, molds, and microalgae [8] (Figure1). FigureFigure 1. SchematicSchematic comparing comparing average average sizes sizes of of the the microorganisms microorganisms described in this review. In addition, some microorganisms have show shownn the capability to biosynthesize unique nanostructured materials, i.e., biom biomineralizedineralized nanostructures like silicified silicified frustules [9], [9], calcifiedcalcified coccoliths [10], [10], magnetosomes [11], [11], and organic nanomaterialsnanomaterials like bacterial nanocellulose [12] [12] exopolysaccharide nanoparticlesnanoparticles [13 ][13] and bacterialand bact nanowireserial nanowires [14]. The microbial-mediated[14]. 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In order engineering to enable a broad tools applicability[15]. In order of microbial-mediatedto enable a broad applicability biosynthesis of microbial-mediatednanomaterials as a real biosynthesis alternative of to ‘traditional’nanomaterials synthetic as a real approaches alternative to nanomanufacturing, to ‘traditional’ synthetic many approacheshurdles still needto nanomanufacturing, to be overcome: a reductionmany hurdles of polidispersity still need ofto nanoparticles,be overcome: aa more reduction complete of polidispersitycharacterization of of nanoparticles, biocapping layer a more agents, complete effectiveness characterization of removal procedures of biocapping of biocapping layer agents, layer effectivenessand nanomaterials of removal purifications, procedures standardization of biocap of microbialping layer cell cultureand nanomaterials protocols for reproducibilitypurifications, standardizationof nanosynthesis of processess,microbial cell as culture well as protocols production for reproducibilit costs and yields.y of nanosynthesis Overeaching theprocessess, challenge as wellfor the as production development costs of and reliable yields. eco-friendly Overeaching nanotechnologies the challenge for for the nanomaterial development synthesisof reliable iseco- of friendlyutmost importance nanotechnologies for future for nanomaterial exploitations synthesis of broad-impact is of utmost nanostructured-based importance for future technologies exploitations and ofapplications, broad-impact like nanostructured-based innovative optical and electrochemicaltechnologies and (bio) applications, sensoristic deviceslike innovative [16] and optical therapeutic and electrochemicaland diagnostic applications (bio) sensoristic of nanostructured devices [16] materials and therapeutic e.g., for drug and delivery, diagnosticin vivo /inapplications vitro imaging of nanostructuredand development materials of antimicrobial e.g., for anddrug antitumoral delivery, in drugs vivo/in [17 ,vitro18]. Inimaging the first and part development of this review, of weantimicrobial reported an and overview antitumoral of scientific drugs literature[17,18]. In (mainly the first from part theof this last review, ten years) we about reportedin vivo an overviewmicrobial ofbiosynthesis scientific literature of nanomaterials (mainly thatfrom have the beenlast ten used years) for (bio) about sensoristic in vivo andmicrobial biomedical biosynthesis purposes. of Wenanomaterials focused on worksthat have that been