## 1 Introduction  
The past two decades have witnessed significant advancement in research efforts to incorporate synthetic short-chain ssDNAs into a variety of applications and miniaturized devices, such as oligonucleotide-Chip technology [1; 2; 3; 4; 5]. In the miniaturization of devices made of nucleic acids, single-stranded DNAs are the preferred choice, which are largely synthesized autonomously or non-autonomously via a staged process that uses conventional phosphoramidite chemistry [6]. The rapid growth in the use of synthetic oligonucleotides made chemical synthesis (3\({}^{\prime}\)-5\({}^{\prime}\) direction) the standard protocol for preparing oligos using a commercial DNA synthesizer. This occurred because of the easy accessibility of the corresponding protected monomeric building blocks [7]. On the other hand, reverse chemical synthesis (5\({}^{\prime}\)-3\({}^{\prime}\) direction) has been limited to nuclease resistance and hairpin loop applications, where the terminal linkage of natural 5\({}^{\prime}\)-3\({}^{\prime}\) antisense oligonucleotides was modified via the construction of alternating 3\({}^{\prime}\)-3\({}^{\prime}\) or 5\({}^{\prime}\)-5\({}^{\prime}\) internucleotide linkages (terminal capping), and hence has not been matured much yet [8; 9; 10; 11]. However, addressed micro-arrays of surface-bound ssDNA (5\({}^{\prime}\)-3\({}^{\prime}\)) with a free 3\({}^{\prime}\) terminal group recently found multiple biotechnology applications in genetics, for example, in enzymatic extension via DNA or RNA polymerase at the 3\({}^{\prime}\) hydroxyl group, and transcriptomics/genomic analysis of cells using 3\({}^{\prime}\)-poly T tail-based primers (5\({}^{\prime}\)-3\({}^{\prime}\)) containing short-chain oligos as a barcode address-tag (<12 bases) [12; 13]. In later applications, researchers have been preparing random sequences for barcode-tags using mix and pool synthesis, which limits the application. Direct reverse synthesis of a specific sequence as an address-tag is attractive to overcome this limitation. Our work presents an application of reverse synthesis which centers upon on-chip and on-demand synthesis of a barcode as an address-tag which can be used for applications like positional information-based transcriptomics and proteomics analysis of single cells. To develop a system for the reverse synthesis of oligonucleotides, microfluidic devices were the favored choice because of their advantages like precise and reproducibly actuate fluids, low reagent consumption, and plentiful quantity production [14]. Most of the microfluidic systems have been limited to continuous flow reactors fabricated in glass or silicon because a variety of organic reagents are needed for chemical syntheses, which limits the workable complexity and reagent savings [15]. In some cases where solvents are mild, it has been possible to use integrated elastomeric material to perform batch syntheses at the nanogram scale [16; 17]. Hua et al. developed a chemically resistant microreactor made of a hybrid silicon-elastomer microfluidic system, but with limited chemical characterizations of the products [18], whereas Quake et al. reported a photocurable perfluoropolyether elastomer for oligonucleotide synthesis (5'-3') [19; 20]. In addition, Quake et al. succeeded in developing mild reagents, which made DNA synthesis feasible on a polydimethylsiloxane- (PDMS-) based microfluidic device [21]. Despite significant advancements, the necessity of an inert atmosphere, skilled operators, and expensive controller units to perform the synthesis is still a significant challenge. Therefore, the development of an efficient system with a simple design and ample product yield under ambient conditions is necessary; this will not only reduce the cost of the process, but also facilitate on-chip and on-demand processing at a laboratory scale.