Spots for individual assays made of nanopaper-based composites on a glass slide. (Image: Eden Morales-Narváez, Institut Català de Nanociencia i Nanotecnologia)

Nanocellulose has been explored in various fields, including filtration (“Nanocellulose filter cleans dirty industry“), wound dressing, as replacement for toxic dyes in textile or security applications, as sponges to combat oil pollution, or as substrate material for flexible and transparent electronics. 

Another significant area for nanopaper applications are sensors. Paper-based sensors promise to be simple, portable, disposable, low power-consuming, and inexpensive sensor devices that will find ubiquitous use in medicine, detecting explosives, toxic substances, and environmental studies. 

“To date, bacterial nanopaper has been scarcely explored for optical (bio)sensing applications,” Arben Merkoçi, ICREA Research Professor and director of the Nanobioelectronics & Biosensors Group at Institut Català de Nanociencia i Nanotecnologia, tells Nanowerk. “Hence, we sought to design, fabricate, and test simple, disposable and versatile sensing platforms based on this material.” 

In a new paper, published in the July 2, 2015 online edition of ACS Nano (“Nanopaper as an Optical Sensing Platform”), Merkoçi and his team describe various nanopaper-based nanocomposites that exhibit plasmonic or photoluminescent properties that can be modulated using different reagents. 

“For the first time we report various nanopaper-based optical sensing platforms and describe how they can be tuned, using nanomaterials, to exhibit plasmonic or photoluminescent properties that can be exploited for sensing applications,” say Eden Morales-Narváez, a postdoctoral researcher in Merkoçi’s group and Hamed Golmohammadi, the paper’s first authors. “We also describe several nanopaper configurations for simple devices, including cuvettes, plates and spots that we printed or punched on bacterial cellulose nanopaper.”

The sensing platforms include a colorimetric-based sensor based on nanopaper containing embedded silver and gold nanoparticles; a photoluminescent-based sensor, comprising CdSe@ZnS quantum dots nanocrystals conjugated to nanopaper (which can be photoexcited using UV-visible light); and a potential up-conversion sensing platform constructed from nanopaper functionalized with NaYF4:Yb3+@Er3+&SiO2 nanoparticles (which can be photoexcited using infrared light). 

Figure 1. Schematic of the proposed nanopaper-based composites. (A and B) Fabrication of plasmonic nanopaper: (A) silver nanoparticle/bacterial cellulose nanopaper conjugate (AgNP-BC); (B) gold nanoparticle/bacterial cellulose nanopaper conjugate (AuNP-BC). (C and D) Fabrication of photoluminescent nanopaper: (C) streptavidin-coated CdSe@ZnS quantum dot/bacterial cellulose nanopaper conjugate (QD-BC); (D) aminosilica-coated NaYF4:Yb3+@Er3+&SiO2 up-conversion nanoparticle/ bacterial cellulose nanopaper conjugate (UCNP-BC). (Reprinted with permission by American Chemical Society)

Morales-Narváez explains that the proposed nanopaper-based composites can be obtained using different pathways (depicted in Figure 1 above) including by exploiting the hydroxyl-containing groups of the bacterial cellulose as a reducing agent for chemical reduction of noble metal ions to metal nanoparticles (Route A; see Figure 1A); by adding bacterial cellulose as a nanonetwork to embed metallic nanoparticles during their synthesis (Route B; see Figure 1B); and by producing surface carboxylic groups on the cellulose, for subsequent coupling with protein/amino-functionalized nanoparticles (Route C; Figure 1C,D). 

The team explored modulation of the plasmonic or photoluminescent properties of these platforms using various model biologically relevant analytes (the drug methimazole, the toxic compounds thiourea and cyanide, and iodide).

Moreover, the scientists demonstrate that bacterial cellulose nanopaper is an advantageous preconcentration platform that facilitates the analysis of small volumes of optically active materials (∼4 µL).