Brief description of the innovation: soft-lithography is a lithography-based technique vastly considered a gold standard for fabricating polymeric microfluidic devices at a laboratory scale. it is a method of replica molding wherein a mold is created, and the polymer polydimethylsiloxane (pdms) gets the shape of the mold on polymerization. pdms-based fabrication is highly popular in research due to its cost-effectiveness in producing transparent and biocompatible devices. however, the microchannels fabricated by patterning onto pdms require substrates (i.e. glass, polymers) to seal the polymer from one end to complete the working microfluidic device. the fabrication approach limits itself to bonding or adhesion issues leading to leakages and the difficulties in alignments of inlets and outlets to the fabricated device. the complexity and requirement of human interventions in developing microchannels limit the automation for industrial mass fabrications. in addition, utilization of these devices at higher volumetric flow rates (i.e., higher injection pressures) leads to the destruction of the device, which is highly discouraged due to its 2d patterning conditions. the above issues require an approach that encourages automation via minimized human interventions, no bonding issues (i.e., sealing), and zero micro-scale alignments. in this context, 3d printing (additive manufacturing) provides an alternative solution to develop monolithic microchannels via stereolithography (sla). during the sla process, a computer-aided (cad) model is required to develop the end product, followed by postprocessing (i.e., cleaning and curing). sla printing utilizes a photocurable resin material (i.e., photosensitive polymer (psp)). the psp is irradiated with a concentrated uv laser beam of micron diameter spot size that initiates the photo polymerizations in the monomers leading to the solidification of layers. the primary advantage of sla printing is to realize 3-dimensional microchannels, allowing higher volumetric flow rates due to increased areal densities. 3d printing technology with our design allows us to print layer-by-layer microchannel arrangements with thin excess material, assisting in minimizing material wastage. significantly, soft lithography for developing microchannels consumes more considerable resources than required, limiting the economic benefits of industrial mass fabrications. the invention discloses a methodology that minimizes material wastages, allows higher volumetric flow rates, and integrates microchannels without the requirements for device assembling. the invention proposes a fabrication approach of monolithic multilayered microchannels (mmmd) in a single device via assembly-free 3d printing. the 3d printed devices contain multiple layers of microchannels, i.e., full utilization of printing material and the inherent advantages of 3d printing. here, single-layer, two-layer, three-layer, and four-layer devices have been demonstrated. each layer represents the presence of microchannels, which is not limited by the no. of layers in this methodology. advantages (dual benefits): 1. process of adhering mechanical strength and rigidity to the device in comparison to conventional process and 2. the complete utilization of printing material (i.e., zero wastage) by the creation of multilayered internal microchannels. the proposed methodology also infuses the miniaturization of the microfluidic device for the same, compared to conventional fabrication approaches. the developed mmm four-layered device was able to synthesize silver and gold nanoparticles in less than a second. the prepared precursor and reagent solutions were injected via syringe pumps into the device. laminar flow conditions and serpentine channels facilitated the mixing of reagents, thereby producing nanoparticles. the obtained nanoparticles (2 nm to 10 nm in size) are fully characterized and were in complete accordance with the state of the art. further, the proposed innovation is a newer process to obtain stable nanoparticles in less than a second, to the best of our knowledge. further, the near future needs such ultrafast and affordable processes to meet diversified applications.