Conversion failed when converting the varchar value to data type - sql-server-2008-r2

Question

i search a lot but didn't find what i want this question is ask by many users here but i didn't find my solution so i ask this question.
i have page in which i have Assign and withdraw button which
is used to Assign and withdraw device id for the selected group
id (group id fetch from dropdownlist) from one list-box to another
that part is completed
Now when i add another device id for the same group id i need to
append that updated record and show it in listbox2.
here is my code:
string queryString1 = "select device_id from device_group_master where group_id='" + ddlGroupName.SelectedValue.ToString() +"'";
SqlDataAdapter newdataAdapter = new SqlDataAdapter(queryString1, con);
SqlCommandBuilder newcommandBuilder = new SqlCommandBuilder(newdataAdapter);
newdataAdapter.Fill(ds1, "table");
if (ds1.Tables["table"].Rows.Count != 0)
{
if (ds1.Tables["table"].Rows[0][0].ToString() == "0")
{
string queryString2 = "update DEVICE_GROUP_MASTER set DEVICE_ID='" + device.ToString() + "', NOT_EXCLUDE_DEVICE_ID='" + enable + "' where GROUP_ID='" + ddlGroupName.SelectedValue.ToString() + "'";
SqlCommand update = new SqlCommand(queryString2, con);
update.CommandType = CommandType.Text;
update.ExecuteNonQuery();
}
else
{
string append;
append = ds1.Tables["table"].Rows[0][0].ToString();
append += ","+ device;
string queryString2 = "update DEVICE_GROUP_MASTER set DEVICE_ID='" + append.ToString() + "' where GROUP_ID='" + ddlGroupName.SelectedValue.ToString() + "'";
SqlCommand update = new SqlCommand(queryString2, con);
update.CommandType = CommandType.Text;
update.ExecuteNonQuery();
}
}